[add] fix: return empty string for content under strict mode (#4683) (#10986)

* [add] fix: return empty string for content under strict mode (#4683)

`content: null` doesn't satisfy `ComplexPluginOutput.content: string`
once `strictNullChecks` is on. Core already treats null and '' the same
(`result.content || ''`), so the generated output doesn't change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PjJDfAYR1JPdgiQRvvZZy

eddeee888:oss:issue-fix

* [add] chore: describe output change in changeset (#4683)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PjJDfAYR1JPdgiQRvvZZy

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: eddeee888

.changeset/quiet-pens-return.md

@@ -0,0 +1,5 @@
+---
+'@graphql-codegen/add': patch
+---
+
+The plugin output's `content` is now an empty string (`''`) instead of `null` when `placement` is `prepend` or `append`. This matches the `ComplexPluginOutput` type (`content: string`). Code that calls the plugin directly and checks `content === null` should check for an empty string instead. Generated files are unchanged.

packages/plugins/other/add/src/index.ts

@@ -25,7 +25,7 @@ export const plugin: PluginFunction<AddPluginConfig> = async (
   }
 
   return {
-    content: null,
+    content: '',
     [placement]: Array.isArray(content) ? content : [content],
   };
 };