HLA: improve resiliency to problem HLA files
HLA: improve resiliency to problem HLA files

During CWL runs None/null files can get passed in different ways. This
ensures that we skip hla typing if files matching our expected naming
are not found. Thanks to @lijiayong

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