[BUGFIX] NULL as default for multiple select attribute

The default return value of the getMultiselectForField() method in the
Field domain model is changed from an empty string to NULL.

This makes sure that the name of single select fields will no longer
end with array brackets [] when the select is not multi valued.

Resolves: #60921
Releases: 2.1
Change-Id: Ic305f63024b8b5e0b8033f1c80170c7da2d60cd9