Two concatenated separators are creating null values in list

Default separators in Static List xobjects are " ", "," and "|", and the default behaviour before XWiki 12.0RC1 was to not consider two following separators. For example, we were able to specify list values such as

Foo, Bar

producing the values Foo and Bar, even if two separators were used.
I don't know if this behaviour was expected or coincidental, but it existed. However we refactored the parsing of Static List as part of XWIKI-16968, and the snippet above now produces three values: Foo, null and Bar.
The null values being produced by the fact that we got a separator, then nothing, then another separator.

I guess that we should skip those null values to preserve the old behaviour.