Better null check in normalizeSchema (#22671)

* uncomment exit

* better null check when normalizing schema

Author: wing328

bin/generate-samples.sh

@@ -68,6 +68,5 @@ fi
 
 if [[ -n "$GENERATE_ERROR" ]]; then
   echo "Found exception(s) when running the generator(s) to update the samples."
-  # UNCOMMENT LATER
-  #exit 1
+  exit 1
 fi

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

@@ -696,7 +696,7 @@ protected boolean isSelfReference(String name, Schema subSchema) {
      */
     public Schema normalizeSchema(Schema schema, Set<Schema> visitedSchemas) {
         // normalize reference schema
-        if (StringUtils.isNotEmpty(schema.get$ref())) {
+        if (schema != null && StringUtils.isNotEmpty(schema.get$ref())) {
             normalizeReferenceSchema(schema);
         }