[openapi-normalizer] Fix nullable boolean check in oneOf schema (#15276)

* fix nullable boolean check in oneof

* minor fix

* fix spec

modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java

@@ -691,7 +691,7 @@ public class OpenAPINormalizer {
 
             // if only one element left, simplify to just the element (schema)
             if (schema.getOneOf().size() == 1) {
-                if (schema.getNullable()) { // retain nullable setting
+                if (Boolean.TRUE.equals(schema.getNullable())) { // retain nullable setting
                     ((Schema) schema.getOneOf().get(0)).setNullable(true);
                 }
                 return (Schema) schema.getOneOf().get(0);

modules/openapi-generator/src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml

@@ -55,9 +55,27 @@ components:
         number:
           anyOf:
             - $ref: '#/components/schemas/Number'
+    ParentWithOneOfProperty:
+      type: object
+      properties:
+        number:
+          oneOf:
+            - $ref: '#/components/schemas/Number'
+    ParentWithPluralOneOfProperty:
+      type: object
+      properties:
+        number:
+          oneOf:
+            - $ref: '#/components/schemas/Number'
+            - $ref: '#/components/schemas/Number2'
     Number:
       enum:
         - one
         - two
         - three
+      type: string
+    Number2:
+      enum:
+        - one
+        - two
       type: string