fix NPE in simplifyOneOfAnyOf (#15142)

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

@@ -728,7 +728,7 @@ public class OpenAPINormalizer {
 
             // if only one element left, simplify to just the element (schema)
             if (schema.getAnyOf().size() == 1) {
-                if (schema.getNullable()) { // retain nullable setting
+                if (Boolean.TRUE.equals(schema.getNullable())) { // retain nullable setting
                     ((Schema) schema.getAnyOf().get(0)).setNullable(true);
                 }
                 return (Schema) schema.getAnyOf().get(0);

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

@@ -36,4 +36,11 @@ components:
       enum:
         - A
         - B
+    SingleAnyOfTest:
+      description: to test anyOf (enum string only)
+      anyOf:
+        - type: string
+          enum:
+            - A
+            - B
 

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

@@ -42,4 +42,10 @@ components:
         - type: integer
         - type: string
         - $ref: null
-
+    SingleAnyOfTest:
+      description: to test anyOf (enum string only)
+      anyOf:
+        - type: string
+          enum:
+            - A
+            - B