[foreign-abi] RFR: 8239780: FunctionDescriptor factories and combinators should check whether return layout is null [v2]

[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

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Add FunctionDescriptor API tests

-------------

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/330/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/330/files/c32fb21a..b32b0544

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=330&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=330&range=00-01

  Stats: 86 lines in 1 file changed: 86 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