Fix empty arguments for parameterized tests

[strangelookingnerd]

See jenkinsci/plugin-pom#1270

Fixes empty arguments for @ParameterizedTest in InjectedTest.
In case no .jelly or .properties file is present, there is nothing to check so the tests can be skipped. This needs some special handling in JUnit.

Testing done

As I was totally unaware of the underlying issue when implementing #1132 I did quite a lot of manual testing now to verify the intended behavior. I successfully ran the failing tests mentioned jenkinsci/plugin-pom#1270 as well.
I actually just learned about the fact that this is causing a test to fail rather than to to be automatically skipped by JUnit. Seems like an odd decision to me but kind of fits in the scheme of how JUnit Jupiter works.

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