[jdk21u-dev] RFR: 8314975: JavadocTester should set source path if not specified

[Goetz Lindenmaier]

I backport this for parity with 21.0.4-oracle.

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Commit messages:
 - Backport 2f4d601a56bf02db2766d1f8a2ddfff906b78678

Changes: https://git.openjdk.org/jdk21u-dev/pull/461/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=461&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314975
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/461.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/461/head:pull/461

PR: https://git.openjdk.org/jdk21u-dev/pull/461