RFR: 8273826: Correct Manifest file name and NPE checks

[Hai-May Chao]

On Wed, 6 Oct 2021 16:58:51 GMT, Sean Coffey <coffeys at openjdk.org> wrote:

> Use correct manifest file name in the Manifest verifier checks. 
> Also - extra null check
> 
> The test doesn't reproduce the exact issue reported but should prevent future regressions in this area.

Looks good.

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Marked as reviewed by hchao (Committer).

PR: https://git.openjdk.java.net/jdk/pull/5841