fix: Correctly handle selectionDefault initialization and normalizati…

[enjeck]

…on in SelectionMultiForm

to reproduce the bug this fixes

• Create new column with type multiple selection and define a value as default.
• Then save.
• All rows now receive the default value according to the definition.
• Now open the column again to edit.
• The default value is not checked any more
• When I selected the original value again as the default, the default value was then entered twice in the field
• If I add a new row, the same value is entered three times in the field, even though in edit mode it is not selected at all.