[jdk13u-dev] RFR: 8273826: Correct Manifest file name and NPE checks

[Yuri Nesterenko]

I need to backport this fix for parity with other releases and as a part of backports chain. Some context adjustments were necessary. The relevant tests run fine.

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Commit messages:
 - Backport 03a8d342b86e720d3cba08d540182b4ab161fba3

Changes: https://git.openjdk.java.net/jdk13u-dev/pull/344/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk13u-dev&pr=344&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273826
  Stats: 139 lines in 5 files changed: 128 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk13u-dev/pull/344.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk13u-dev pull/344/head:pull/344

PR: https://git.openjdk.java.net/jdk13u-dev/pull/344