Don't use v:null in core code
Don't use v:null in core code

Fix #563.

OmniSharp.vim is code still runs on old versions of vim, so avoid using
v:null to maintain compatibility with vim 7.4.

v:null also appears in stdio.vim, but that feature requires more recent
vim, so it's fine.

OmniSharp/omnisharp-vim,OmniSharp/omnisharp-vim,OmniSharp/omnisharp-vim