fixed elements improperly create non-empty default
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See: https://sourceforge.net/p/pyxb/discussion/956708/thread/b6c1884d
When #99 was fixed the reset value for an element was changed to the default values of its element content; prior to that element content was reset to None. The new reset doesn't take into account that a non-empty element may not be valid.
fix for issue (may have whitespace errors)
Fixed in following commit on next branch.