[jdk21u-dev] RFR: 8305971: NPE in JavacProcessingEnvironment for missing enum constructor body

[Aleksey Shipilev]

Clean backport to resolve javac crash. The patch is simple, and fixes the path where the alternative is NPE.

Additional testing:
 - [x] New test fails without the fix, passes with it
 - [x] `langtools_all` still passes

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Commit messages:
 - Backport 01ea1eff66e43f106640ecfd19fadf2c8245a1ad

Changes: https://git.openjdk.org/jdk21u-dev/pull/25/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=25&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305971
  Stats: 132 lines in 2 files changed: 131 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/25.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/25/head:pull/25

PR: https://git.openjdk.org/jdk21u-dev/pull/25