[foreign-memaccess+abi] RFR: 8301806: TestNulls does not cover all API classes

[Jorn Vernee]

The TestNulls test does not cover the following API classes: Linker.Option, PaddingLayout, StructLayout, UnionLayout.

This patch adds coverage, and fixes several places where we weren't checking for `null`.

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Commit messages:
 - add more coverage
 - check for nulls

Changes: https://git.openjdk.org/panama-foreign/pull/787/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301806
  Stats: 12 lines in 2 files changed: 10 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/787.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/787/head:pull/787

PR: https://git.openjdk.org/panama-foreign/pull/787