False positive in Resource Leak Checker when type variable has a non-empty MustCall type

[kelloggm]

The test https://github.com/typetools/checker-framework/blob/master/checker/tests/resourceleak/Issue4815.java demonstrates a false positive from the Resource Leak Checker. I don't know why this warning is issued, because the code looks safe to me. This false positive should be investigated and fixed.

This is a continuation of #4815, which reported a crash from the RLC on that test case. #4821 fixed the crash, but the false positive remains.