RFR: 8323571: Regression in source resolution process

[Joe Wang]

Fix a regression in the source resolution process where it failed to recognize a custom InputSource when both the public and system IDs are null. The particular issue is fixed at line 1233, the rest of the changes is formatting to make the null-check on InputSource the first.

Test is provided by Marc, thanks Marc! (https://github.com/junit-team/junit5/issues/3594)

XML tests, including the new test, pass with this change.

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Commit messages:
 - 8323571: Regression in source resolution process

Changes: https://git.openjdk.org/jdk/pull/17377/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17377&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323571
  Stats: 355 lines in 4 files changed: 339 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/17377.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17377/head:pull/17377

PR: https://git.openjdk.org/jdk/pull/17377