[Java][Haskell][Python] fix errors with empty strings, missing basePath

[martin-mfg]

I used this simple OpenApi file as input for all generators. This uncovered problems in several generators. Some threw an Exception while processing the input, while others generated code that doesn't compile. Most generators couldn't handle the empty strings in this input.

I upgraded quarkus because with the old version, compilation failed with

[error]: Build step io.quarkus.resteasy.deployment.ResteasyServletProcessor#jaxrsConfig threw an exception: java.lang.IllegalArgumentException: Path must start with /

This is because there is no servers section in the input file, and thus no base path. The new version doesn't have this problem.

In this PR, I fixed all discovered problems.

It might also make sense to add tests based on the mentioned OpenApi input file. But I don't see a good way of using one input file with all generators. And I didn't want to add one new file for each generator to /bin/configs, because that might be too much. If you have an idea how to add tests for all generators, please let me know.

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[wing328]

thanks for the fix.

cc @OpenAPITools/generator-core-team