to fix loop for allof with single element in normalizer (#17426)

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

@@ -573,6 +573,10 @@ public class OpenAPINormalizer {
             return;
         }
 
+        if (schema.getAllOf().size() == 1) {
+            return;
+        }
+
         for (Object item : schema.getAllOf()) {
             if (!(item instanceof Schema)) {
                 throw new RuntimeException("Error! allOf schema is not of the type Schema: " + item);

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -2022,7 +2022,7 @@ public class JavaClientCodegenTest {
         generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "true");
         List<File> files = generator.opts(clientOptInput).generate();
 
-        Assert.assertEquals(files.size(), 24);
+        Assert.assertEquals(files.size(), 27);
         validateJavaSourceFiles(files);
 
         TestUtils.assertFileContains(

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

@@ -96,4 +96,9 @@ components:
         mum_or_dad:
           type: string
       required:
-        - isParent
+        - isParent
+    allOfWithSingleItem:
+      description: allOf with a single item
+      nullable: true
+      allOf:
+        - $ref: '#/components/schemas/AnotherParent'