make datatype mapping more robust

Author: nr23730

src/main/java/de/uksh/medic/etl/openehrmapper/Generator.java

@@ -706,7 +706,7 @@ public void gen_DV_CODED_TEXT(String path, String name, Object jsonmap,
             default -> {
             }
         }
-        if (map.containsKey("mapping_" + name)) {
+        if (map.containsKey("mapping_" + name) && map.get("mapping_" + name) != null) {
             List<DatatypeMapping> ldm = (List<DatatypeMapping>) map.get("mapping_" + name);
             for (DatatypeMapping dm : ldm) {
                 TermMapping tm = new TermMapping();