RFR: 8325090: javadoc fails when -subpackages option is used with non-modular -source [v2]

[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.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Align indentation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21201/files
  - new: https://git.openjdk.org/jdk/pull/21201/files/cbca091d..9e0440f6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21201&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21201&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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