Make the `GroffLexer` play nice with being asked to analyse the empty string
Make the `GroffLexer` play nice with being asked to analyse the empty string

--HG--
branch : trunk

nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments,nex3/pygments