[PHP-Symfony] Fix handling of null date-time parameter

This fixes one of the issue described on #14930, namely that
the deserializeString method of the generated class JsmSerializer returns null
for other types of string, but not for date-time. Instead it returns a DateTime
which represents "now" (because that what `new DateTime(null)` does).

Consequently when an API declares a date-time parameter as non-required and
when that endpoint is called without that parameters, then the user code
would end up having "now" instead of "null" for this parameter.

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

@@ -95,7 +95,7 @@ class JmsSerializer implements SerializerInterface
                 break;
             case 'DateTime':
             case '\DateTime':
-                return new DateTime($data);
+                return is_null($data) ? null :new DateTime($data);
             default:
                 throw new RuntimeException(sprintf("Type %s is unsupported", $type));
         }

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

@@ -95,7 +95,7 @@ private function deserializeString($data, string $type)
                 break;
             case 'DateTime':
             case '\DateTime':
-                return new DateTime($data);
+                return is_null($data) ? null :new DateTime($data);
             default:
                 throw new RuntimeException(sprintf("Type %s is unsupported", $type));
         }