fix problem with sending empty remote branch name in push command

In one place ```if (requireBranch != null)``` construction was used instead of ```if (requireBranch)``` what causes:
```
00:02:40  >>> Running [git, --git-dir=/home/foo/bar/.git, --work-tree=/home/foo/bar, push, origin, ]: [][fatal: remote part of refspec is not a valid name in 
```
for requireBranch parameter set to ''.