Treat empty-declaration-subset as no-declaration-subset and allow DTD caching

Running Xerces with the following features switched on:

• http://apache.org/xml/features/validation/use-cachedGrammarInParse
• http://apache.org/xml/features/validation/cache-grammarFromParse
• http://apache.org/xml/features/validation/ignoreCachedDTD

on a document containing an empty declaration subset returns error:

"When reusing the Grammar, no internal subset is allowed"

I would prefer if an empty internal declaration subset would be treated as a missing internal declaration subset.
I could submit a patch here if that would help.

Thanks