RFR: 8325090: javadoc fails when -subpackages option is used with non-modular -source

[Hannes Wallnöfer]

Please review a simple fix to prevent `java.lang.AssertionError` to be thrown when the `javadoc` `-subpackages` option is used with a `-source` release that doesn't support modules.

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Commit messages:
 - JDK-8325090: javadoc fails when -subpackages option is used with non-modular -source

Changes: https://git.openjdk.org/jdk/pull/21201/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21201&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325090
  Stats: 82 lines in 2 files changed: 77 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21201.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21201/head:pull/21201

PR: https://git.openjdk.org/jdk/pull/21201