RestEasy Reactive client: optional FormParameters throw error when null

[pravussum]

Describe the bug

When using a reactive restclient created from an interface, it's not possible to call a method with an optional form parameter when the actual value of the parameter is null.
It gives the following error:

java.lang.IllegalStateException: Form parameter 'formParam' could not be converted to 'String' for REST Client interface 'org.acme.GreetingResourceApi'. A proper implementation of 'javax.ws.rs.ext.ParamConverter' needs to be returned by a 'javax.ws.rs.ext.ParamConverterProvider' that is registered with the client via the @RegisterProvider annotation on the REST Client interface.

Expected behavior

The service method can be called without error and the value of the delivered parameter value is null.

Actual behavior

The error message from the bug description.

How to Reproduce?

Clone the following reproducer project:
https://github.com/pravussum/quarkus-restclient-reactive-formparam-repro

The test ReactiveGreetingResourceTest shows the good case (form param not null) and the error case (sent form param is null).

Output of uname -a or ver

5.13.0-27-generic #29-Ubuntu SMP Wed Jan 12 17:36:47 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Output of java -version

openjdk-15.0.2

GraalVM version (if different from Java)

No response

Quarkus version or git rev

2.7.1

Build tool (ie. output of mvnw --version or gradlew --version)

Maven 3.8.4

Additional information

The JaxrsClientReactiveProcessor.addFormParam() method does a "isString" check on the null value returned from the converter, which obviously fails and leads to the given error message.
The null case should be handled properly here (neither throwing an exception, nor adding the parameter to the formParams map).

[quarkus-bot]

/cc @FroMage, @geoand, @stuartwdouglas