[TASK] Placeholder attribute without useOrOverridePlaceholder

The placeholder configuration for a TCA field will now be considered
independently from the "mode" beeing set to "useOrOverridePlaceholder".
If a placeholder is set in the TCA it will be rendered. The mode
setting only changes the behavior of the NULL checkbox.

Relsolves: #59129
Releases: 6.3
Change-Id: Ice7681d644623a5e94cbdd22743ddf5aab8919a9