[BUG] Symfony4 StrictJsonDeserializationVisitor::visitString declaration incompatible

Created by: BenjaminHae

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When generating the code for php-symfony it does not work as intended with symfony 4.2.2. Running

php bin/console server:start 0.0.0.0:8000

and visiting an registered endpoint leads to the following fatal error:

(1/1) FatalErrorException
Compile Error: Declaration of OpenAPI\Server\Service\StrictJsonDeserializationVisitor::visitString($data, array $type, JMS\Serializer\Context $context) must be compatible with JMS\Serializer\JsonDeserializationVisitor::visitString($data, array $type): string
--
in StrictJsonDeserializationVisitor.php line 23

openapi-generator version

master, v4.0.0-beta, latest

Command line used for generation

docker run --rm -v ${PWD}:/local test/openapi-generator-cli generate -i /local/swagger.yaml     -g php-symfony -o /local/4Symfony

Steps to reproduce

Implement a symfony4 application by doing step 1 through 5 of the README. Then apply the fix for private controllers: https://github.com/OpenAPITools/openapi-generator/pull/1003 Start the server

php bin/console server:start 0.0.0.0:8000

and visit an endpoint.