RFR: 8321406: Null IDs should be resolved as before catalogs are added

[Joe Wang]

Improve null handling. When both the public and system IDs are null, this change lets the resolution process continue as usual, before the catalogs were added. This is consistent with the process on the other part of the program where a CatalogResolver is used only when the IDs are not null.

Test: existing XML tests passed.

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Commit messages:
 - 8321406: Null IDs should be resolved as before catalogs are added

Changes: https://git.openjdk.org/jdk/pull/16983/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16983&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321406
  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16983.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16983/head:pull/16983

PR: https://git.openjdk.org/jdk/pull/16983