RFR: 7903673: Jextract throws NPE when generating enum constant javadoc [v2]

[Maurizio Cimadamore]

> This PR fixes `TreeMaker` so that it doesn't try to generate javadoc for `null` enum constants.
> 
> Unfortunately I was not able to reproduce on Linux. The problem seems to appear on MacOS when extracting this enum:
> 
> 
> enum __CFByteOrder {
>     CFByteOrderUnknown,
>     CFByteOrderLittleEndian,
>     CFByteOrderBigEndian
> }; 
> 
> 
> But there doesn't seem to be anything peculiar with this (and this does work when extracted in isolation).
> It might indicate a deeper issue in libclang. For now, I made the code more robust, so that it handles possible null enum constants (these are left out from the IR).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comment

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/219/files
  - new: https://git.openjdk.org/jextract/pull/219/files/33af9395..ef0fa41d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=219&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=219&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/219.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/219/head:pull/219

PR: https://git.openjdk.org/jextract/pull/219