[foreign-abi] RFR: 8239780: FunctionDescriptor factories and combinators should check whether return layout is null

[Jorn Vernee]

Hi,

I'm going through the foreign-abi JBS issues and taking care of some of the open ones.

This PR adds null checking to the MemoryLayouts passed to the FunctionDescriptor factories and adapter methods,
together with a simple test.

I've also added an adapter for dropping the return layout, since with the null check, it is no longer possible to pass
`null` to drop the return layout. I think having the explicit check, and a separate combinator for dropping the return
layout instead will be better for avoiding accidental nulls being used.

Thanks,
Jorn

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Commit messages:
 - Add null checks to FunctionDescriptor factories.

Changes: https://git.openjdk.java.net/panama-foreign/pull/330/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=330&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8239780
  Stats: 78 lines in 2 files changed: 78 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/330.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/330/head:pull/330

PR: https://git.openjdk.java.net/panama-foreign/pull/330