[BUGFIX] Allow maxitem=1 in TCA treeSelect again

Introduced with ec7b2297 #75519, it was not possible to set maxitem=1 to
TCA treeSelect. This occures e.g. on setting the parent of a
sys_category.

The reason is the "selected" array which gets initialized with all
selected items. If there is no item selected, it is initialized with an
empty value and "selected.lenght" is 1 but should be 0. Selecting a new
value is impossible now.

The proposed path checks if items are selected or not on
initialization.

Change-Id: I0c8d21ad753dccb2d993febde404afc83a39b9b3
Resolves: #75947
Releases: master, 7.6, 6.2
Reviewed-on: https://review.typo3.org/47940


Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Tested-by: Philipp Wrann <philippwrann@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>