HADOOP-16707. NPE in UGI.getCurrentUser in ITestAbfsIdentityTransformer setup.

[steveloughran]

Contributed by Steve Loughran.

Change-Id: I38fdba2fa70e534d78b15e61de19368912588b0c

Tested: azure wales

[steveloughran]

reviews requested from @lmccay @DadanielZ
Although the problem surfaced in test setup in the ABFS tests (and went away with this patch), the root cause is that's not all paths into UGI make sure that it is fully initialised, and if a test resets it, things can fail.

This fixes the production code rather than the symptoms of the failure.

[hadoop-yetus]

💔 -1 overall

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Docker	Client=19.03.5 Server=19.03.5 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-1723/1/artifact/out/Dockerfile
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[steveloughran]

no new tests as I verified that the failures in ITestAbfsIdentityTransformer went away.

[DadanielZ]

Looks good to me, +1

[steveloughran]

thanks!