[php-symfony] Don't crash at runtime on null convertFormat

$this->convertFormat may return "null". When it's the case we end up
calling

    ...->serialize($data, null);

but this crashes at runtime because that serialize method declares that
the 2nd parameter is of type "string" (so null is not accepted).

With this patch we avoid having an error 500. Instead we return something
that makes perfect sense when the OpenApi specification declares a content
of type "text/plain" and that the returned value is for instance a string,
an int, or a boolean.

Author: gturri

modules/openapi-generator/src/main/resources/php-symfony/serialization/JmsSerializer.mustache

@@ -29,7 +29,12 @@ class JmsSerializer implements SerializerInterface
      */
     public function serialize($data, string $format): string
     {
-        return SerializerBuilder::create()->build()->serialize($data, $this->convertFormat($format));
+       $convertFormat = $this->convertFormat($format);
+       if ($convertFormat !== null) {
+        return SerializerBuilder::create()->build()->serialize($data, $convertFormat);
+       } else {
+         return is_string($data) ? $data : var_export($data, true);
+       }
     }
 
     /**

samples/server/petstore/php-symfony/SymfonyBundle-php/Service/JmsSerializer.php

@@ -29,7 +29,12 @@ public function __construct()
      */
     public function serialize($data, string $format): string
     {
-        return SerializerBuilder::create()->build()->serialize($data, $this->convertFormat($format));
+       $convertFormat = $this->convertFormat($format);
+       if ($convertFormat !== null) {
+        return SerializerBuilder::create()->build()->serialize($data, $convertFormat);
+       } else {
+         return is_string($data) ? $data : var_export($data, true);
+       }
     }
 
     /**