fixes unwanted null values with multiple select

[JoernT]

when selecting values from pb-select multiple an unwanted 'null' value was created in formData.

Using 'undefined' instead fixes the issue.

[tuurma]

don't know how to do that now but commit messages need to follow certain convention now to be picked up correctly for automated release, so yours should read something like: fix(pb-select) fixes unwanted null values with multiple select

[JoernT]

ah thanks - forgot about that

[JoernT]

i'll redo it