[test] Flake8-clean and use stdlib unittest in cubicweb/server/test/unittest_utils.py

[web] Don't raise ProcessFormError if there is no value specified a03376213747 introduced a slight modification in None / empty string values and with it a regression where an exception is raised on empty values, while we should simply consider no value is specified and return None.

[test] Cleanup imports in unittest_form Remove unused and reorder, + use bare unittest along the way.