[TASK] Allow to omit tables in ext_tables.sql

We are auto-creating more and more table columns
from TCA automatically with our works on #101553.
ext_tables.sql can now end up defining no column
of a table at all anymore.

The schema parser currently stumbles when a table
definition is completely left out in ext_tables.sql.

The patch allows omitting a table definition in
ext_tables.sql entirely.

Resolves: #102085
Related: #101553
Releases: main
Change-Id: I4d63250e3baa15e094f011ed21c813bc6cb12fb0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81181


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>