fix(java): preserve null JSON header semantics

Author: AndreyVMarkelov

modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache

@@ -919,6 +919,9 @@ public class ApiClient {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -632,6 +632,7 @@ public void testDynamicJsonContentHeaderUsesJsonSerialization() {
         assertThat(output.resolve("src/main/java/org/openapitools/client/ApiClient.java"))
                 .content()
                 .contains("if (param.getContent() != null && param.getContent().containsKey(\"application/json\")) {")
+                .contains("public String parameterToJsonString(Object param) {\n        if (param == null) {\n            return \"\";\n        }")
                 .contains("headerParams.put(param.getName(), parameterToJsonString(value));")
                 .contains("headerParams.put(param.getName(), parameterToString(value));");
 

samples/client/echo_api/java/okhttp-gson-user-defined-templates/src/main/java/org/openapitools/client/ApiClient.java

@@ -800,6 +800,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/echo_api/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java

@@ -730,6 +730,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/others/java/okhttp-gson-oneOf-array/src/main/java/org/openapitools/client/ApiClient.java

@@ -704,6 +704,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/others/java/okhttp-gson-oneOf/src/main/java/org/openapitools/client/ApiClient.java

@@ -704,6 +704,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java

@@ -704,6 +704,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/petstore/java/okhttp-gson-3.1-duplicated-operationid/src/main/java/org/openapitools/client/ApiClient.java

@@ -800,6 +800,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/ApiClient.java

@@ -800,6 +800,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {

samples/client/petstore/java/okhttp-gson-awsv4signature/src/main/java/org/openapitools/client/ApiClient.java

@@ -820,6 +820,9 @@ public String parameterToString(Object param) {
      * @return JSON representation of the parameter
      */
     public String parameterToJsonString(Object param) {
+        if (param == null) {
+            return "";
+        }
         String json = JSON.serialize(param);
         int firstUnsafe = -1;
         for (int i = 0; i < json.length(); i++) {