[TASK] Unify NULL default value handling

If a field is configured to allow NULL values ("eval" is set to "null")
and the default value in the TCA is explicitly set to NULL the value
will not be converted to a string.

Additionally the mode setting "useOrOverridePlaceholder" will not have
any influence any more on the default value when creating new records.

Resolves: #54412
Releases: 6.2
Documentation: #56584
Change-Id: Ica43a1af33e6f86dd042b66781acc0e9cd904cf9
Reviewed-on: https://review.typo3.org/26408
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Reviewed-by: Oliver Hader
Reviewed-by: Ernesto Baschny
Reviewed-by: Stefan Neufeind
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe