Fix zero value in argument
 Fix empty with long mandatory flag
Merge pull request #3950 from blanchonvincent/hotfix/issue/3851

Fix zero value in argument
Additional test

- Added optional-long-flag-match-with-zero-value test case; passes.
[#3950] CS fixes

- trailing whitespace
Merge branch 'hotfix/3950'

Close #3950
Fixes #3851