[BUGFIX] Extension restdoc is not loaded

The feature introduced with #52347 that allows to automatically
fix a missing Intersphinx mapping requires EXT:restdoc. Make
sure this method does not crash if extension is missing.

Change-Id: I334ad0b4eda1d1842c3259cdb59e3f0bb30fde41
Fixes: #57474
Related: #52347
Reviewed-on: https://review.typo3.org/29008
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers