Fix: handle nullable schemas in ModelUtils and DefaultCodegen

Author: sshekhar563

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

@@ -4043,7 +4043,15 @@ public CodegenProperty fromProperty(String name, Schema p, boolean required, boo
             property.isWriteOnly = p.getWriteOnly();
         }
         if (p.getNullable() != null) {
-            property.isNullable = p.getNullable();
+        if (Boolean.TRUE.equals(p.getNullable())) {
+            property.isNullable = true;
+        } else if (p.getOneOf() != null && p.getOneOf().stream().anyMatch(
+            s -> "null".equals(s.getType()))) {
+            property.isNullable = true;
+        } else {
+            property.isNullable = false;
+        }
+
         }
 
         if (p.getExtensions() != null && !p.getExtensions().isEmpty()) {

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

@@ -2261,9 +2261,20 @@ public static Schema cloneSchema(Schema schema, boolean openapi31) {
      */
     public static Schema simplifyOneOfAnyOfWithOnlyOneNonNullSubSchema(OpenAPI openAPI, Schema schema, List<Schema> subSchemas) {
         if (subSchemas.removeIf(subSchema -> isNullTypeSchema(openAPI, subSchema))) {
-            schema.setNullable(true);
+           if (subSchemas.size() == 1) {
+               Schema<?> nonNull = subSchemas.get(0);
+               schema.setOneOf(Arrays.asList(
+               nonNull,
+               new Schema<>().type("null")
+            ));
+            return schema;
+            } else {
+                    schema.setOneOf(subSchemas);
+                    return schema;
+            }
         }
 
+
         // if only one element left, simplify to just the element (schema)
         if (subSchemas.size() == 1) {
             Schema<?> subSchema = subSchemas.get(0);
@@ -2608,4 +2619,4 @@ public LinkedHashSet<String> build() {
             }
         }
     }
-}
+}