[foreign-abi] RFR: 8256466: MemoryLayout factories do not guard against null layouts

[Maurizio Cimadamore]

This patch fixes the remaining issues with NPE in javadoc and implementation in the foreign-abi branch. For testing only few extra lines in the harness were required, the most difficult part being the VaList/Builder stuff (since these are scopd abstractions which can only be used in a certain way).

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Commit messages:
 - Merge branch 'foreign-abi' into abi-null-checks
 - Rebase on the new NPE harness
 - Merge branch 'null-harness' into abi-null-checks
 - Add combinatorial test to check handling of nulls in reference-accepting APIs
 - Initial tests

Changes: https://git.openjdk.java.net/panama-foreign/pull/399/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=399&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256466
  Stats: 173 lines in 11 files changed: 151 ins; 9 del; 13 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/399.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/399/head:pull/399

PR: https://git.openjdk.java.net/panama-foreign/pull/399