editor and content type properties from TextArea xclass are not reset if initializer set a null value

Calling BaseClass#addTextAreaField with null values (or no values) for editor or contentType properties does not lead to a reset of those properties.

This might be a problem, for example we recently change initializer of XWikiComments to not use Text editor by default, but to leave the editor property unset so that it can fallback on user value.