Update StringTrim to allow '0' as charlist

Update Zend\Filter\StringTrim to allow '0' as charlist
Add Test for empty charlist in StringTrim
Update StringTrimTest.php
Update StringTrimTest.php

Removed trailing spaces
#6261 - minor CS fixes in `StringTrim`
#6261 - adding missing coverage annotations
#6261 - adding missing group annotation
Merge branch 'hotfix/#6261-string-trim-allows-0-in-charlist'

Close #6261