fix(metadata): make the schema attribute to fallback to null for parameters in YamlResourceExtractor

[wuchen90]

Q	A
Branch?	3.4
License	MIT

When we omit the schema attribute for parameters config in YAML resource definition, the YamlResourceExtractor breaks.

Definition like this:

resources:
  Foo:
    operations:
      ApiPlatform\Metadata\GetCollection:
        parameters:
          bar:
            # schema: ...
            property: baz

For DX, the schema attribute should logically fallback to null as \ApiPlatform\Metadata\Parameter::__construct defines it.