Add empty query check to Grep

[jyscao]

I accidentally froze up my entire vim session, then subsequently also my computer session by accidentally executing :Grep without passing a query. I came over from using Grepper, which required executing :Grepper alone to initiate a grepping search, so it must have been muscle memory.

Figured others likely also ran into (or will continue to run into) this problem too, hence this quick and dirty fix.

[jyscao]

Realized my initial fix would've left &l:grepprg (and related) in their modified state without switching them back, so I made the change and force pushed.

(also took the liberty of removing the comments related to mileszs/ack.vim, which look obsolete to me)

[liuchengxu]

Thank you!