Remove whitespace values from content providers

[rfc2822]

Actual problem: event.RDATE is sometimes filled with whitespace like \n when an .ics is directly imported with a calendar app. For instance, Zoom seems to sometimes generate such RDATEs.

Then DAVx⁵ uploads the event with an empty RDATE line like:

RDATE;TZID=Europe/Paris:

and servers reject that with 415.

So I suggest to:

• extend MiscUtils.removeEmptyStrings so that it removes not only empty, but also blank (= only white-space, including newline) values + test
• apply MiscUtils.removeEmptyStrings to populateTask() like it's done for populateEvent() ?