[8u] RFR 8273826: Correct Manifest file name and NPE checks

[Martin Balao]

Hi,

I'd like to request a review for the 8u backport of JDK-8273826 [1].

Webrev.00:

 *
http://cr.openjdk.java.net/~mbalao/webrevs/8273826/8273826.webrev.8u.jdk.00/

The 11u patch does not apply cleanly because:

 * ManifestEntryVerifier.java
  * 8078468 is not in 8u
   * This affects the context only, not the actual code modified. I
don't see it as a blocking dependency. Manually applied the change.

 * LowerCaseManifest.java
  * JarUtils imports and lib path adjusted to 8u location
  * Path.of -> Paths.get replacement

The new 'LowerCaseManifest.java' test passes. No tests regressions in
jdk/sun/security/tools/jarsigner.

Thanks,
Martin.-

--
[1] - https://bugs.openjdk.java.net/browse/JDK-8273826