Make things qork in the case that no foreground is present (#4908)

bin/all_sky_search/pycbc_coinc_statmap

@@ -291,9 +291,9 @@ if fore_locs.sum() > 0:
     f['foreground/ifar_exc'] = conv.sec_to_year(ifar_exc)
     f['foreground/fap_exc'] = fap_exc
 else:
-    f['foreground/ifar'] = numpy.array([])
+    f['foreground/ifar'] = ifar = numpy.array([])
     f['foreground/fap'] = numpy.array([])
-    f['foreground/ifar_exc'] = numpy.array([])
+    f['foreground/ifar_exc'] = ifar_exc = numpy.array([])
     f['foreground/fap_exc'] = numpy.array([])
 
 if 'name' in all_trigs.attrs:
@@ -331,6 +331,10 @@ else :
 #          will happen when ifar_foreground has no elements greater than
 #          the background time.
 
+# This is a workaround for the case that we have no foreground, so that the
+# while loop comparison doesnt fail straight away
+ifar_foreground = numpy.append(ifar_foreground, 0)
+
 while numpy.any(ifar_foreground >= background_time):
     # If the user wants to stop doing hierarchical removals after a set
     # number of iterations then break when that happens.