bugfix - allow non-running of singles (gwastro#4439)

* bugfix - allow non-running of singles

* CC

* Revrt wrongly-added test chnages

Author: GarethCabournDavies

pycbc/events/single.py

@@ -108,6 +108,10 @@ def verify_args(args, parser, ifos):
                 parser.error("All IFOs required in --single-ifar-est-dist "
                              "if IFO-specific options are given.")
 
+            if args.sngl_ifar_est_dist[ifo] is None:
+                # Default - no singles being used
+                continue
+
             if not args.sngl_ifar_est_dist[ifo] == 'fixed':
                 if not args.single_fit_file:
                     # Fixed IFAR option doesnt need the fits file