refactor: Annotate methods which may return null with @Nullable

Use this link to re-run the recipe: https://app.moderne.io/recipes/org.openrewrite.staticanalysis.AnnotateNullableMethods?organizationId=T3BlblJld3JpdGU%3D

Co-authored-by: Moderne <[email protected]>

plugin/src/main/java/org/openrewrite/gradle/RewriteExtension.java

@@ -99,7 +99,7 @@ public void setCheckstyleConfigFile(File configFile) {
      * Will prefer to return an explicitly configured checkstyle configuration file location.
      * If none has been specified, will attempt to auto-detect an appropriate file.
      */
-    public @Nullable File getCheckstyleConfigFile() {
+    public @org.jspecify.annotations.Nullable @Nullable File getCheckstyleConfigFile() {
         if (checkstyleConfigFile == null && checkstyleConfigProvider != null) {
             try {
                 return checkstyleConfigProvider.get();