Remove unreachable null check from BuildClasspathMojo

[wilx]

getResolvedDependencies(true) supplies a non-null Set; projects with no resolved dependencies receive an empty set. Remove the misleading artifacts == null condition from BuildClasspathMojo while retaining the existing empty-set logging and empty classpath output.

This aligns the Mojo with the dependency resolver's effective contract. Because the null case cannot be produced through the supported Maven dependency-resolution path, no defensive conversion or null-specific test is needed.

Fixes #1648.

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[elharo]

needs a test

[elharo]

can this method return null? what's it's contract? and if it can return null, should it be changed to return an empty set instead?

[wilx]

Not in the current Maven, I think. But I did not study all the versions.

[elharo]

Is the method marked nullable or non-nullable?

[wilx]

It is not marked.

[elharo]

I don't see an issue here. If the method doesn;t return null, it doesn't return null.

[wilx]

I have removed the check and reworded the commit.