Use conditional access for enumref query params (#20418)

Author: JulianVennen

modules/openapi-generator/src/main/resources/php-nextgen/api.mustache

@@ -711,7 +711,7 @@ use {{invokerPackage}}\ObjectSerializer;
         {{#queryParams}}
         // query params
         $queryParams = array_merge($queryParams, ObjectSerializer::toQueryValue(
-            ${{paramName}}{{#isEnumRef}}->value{{/isEnumRef}},
+            ${{paramName}}{{#isEnumRef}}?->value{{/isEnumRef}},
             '{{baseName}}', // param base name
             '{{#schema}}{{openApiType}}{{/schema}}', // openApiType
             '{{style}}', // style

samples/client/echo_api/php-nextgen-streaming/src/Api/QueryApi.php

@@ -424,7 +424,7 @@ public function testEnumRefStringRequest(
         ) ?? []);
         // query params
         $queryParams = array_merge($queryParams, ObjectSerializer::toQueryValue(
-            $enum_ref_string_query->value,
+            $enum_ref_string_query?->value,
             'enum_ref_string_query', // param base name
             'StringEnumRef', // openApiType
             'form', // style

samples/client/echo_api/php-nextgen/src/Api/QueryApi.php

@@ -424,7 +424,7 @@ public function testEnumRefStringRequest(
         ) ?? []);
         // query params
         $queryParams = array_merge($queryParams, ObjectSerializer::toQueryValue(
-            $enum_ref_string_query->value,
+            $enum_ref_string_query?->value,
             'enum_ref_string_query', // param base name
             'StringEnumRef', // openApiType
             'form', // style

samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Api/FakeApi.php

@@ -773,7 +773,7 @@ public function fakeEnumEndpointRequest(
 
         // query params
         $queryParams = array_merge($queryParams, ObjectSerializer::toQueryValue(
-            $enum_class->value,
+            $enum_class?->value,
             'enum-class', // param base name
             'EnumClass', // openApiType
             'form', // style