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

[Iris Clark]

On Tue, 5 Dec 2023 19:34:58 GMT, Joe Wang <joehw at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16983#pullrequestreview-1766224226