RFR: 8265418: Clean-up redundant null-checks of Class.getPackageName()

[Сергей Цыпанов]

As discussed in https://github.com/openjdk/jdk/pull/3464 we can clean-up null-checks remaining after [8142968](https://bugs.openjdk.java.net/browse/JDK-8142968) as Class.getPackageName() never returns null.

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Commit messages:
 - 8265418: Clean-up redundant null-checks of Class.getPackageName()

Changes: https://git.openjdk.java.net/jdk/pull/3571/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3571&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265418
  Stats: 19 lines in 7 files changed: 1 ins; 8 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3571.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3571/head:pull/3571

PR: https://git.openjdk.java.net/jdk/pull/3571