[BUGFIX] String TCA Fallback in the Mapper

The Mapper should always return a valid TCA configuration. Also
in case of complex data types. Complex TCA fields have to override
in the given TCA files.

The getDatabaseDefinition method throw still a Exception in case of
a invalid Mapper type.

Change-Id: I7bc01db107ff2a178f2e26e6307f6f60f1a110ce
Reviewed-on: https://review.typo3.org/31532
Reviewed-by: Tim Spiekerkötter
Tested-by: Tim Spiekerkötter