VarScan: avoid grep failure with all 0 output
VarScan: avoid grep failure with all 0 output

Avoids grep returning a 1 exit code when an input is all empty bases
and gets filtered out. grep returns a 1 in this case (no output)
which causes the pipe to fail. Instead we now pass through an empty
result and allow VarScan to continue. Thanks to @hyong2000. Fixes #1199

biocyberman/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,brainstorm/bcbio-nextgen,mjafin/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,mjafin/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,brainstorm/bcbio-nextgen,chapmanb/bcbio-nextgen,mjafin/bcbio-nextgen,biocyberman/bcbio-nextgen,chapmanb/bcbio-nextgen