fix check for custom networking

Author: mr-c

cwltool/singularity.py

@@ -235,7 +235,7 @@ def create_runtime(self,
         runtime.append(u"--pwd")
         runtime.append("%s" % (docker_windows_path_adjust(self.builder.outdir)))
 
-        if runtimeContext.custom_net is not None:
+        if runtimeContext.custom_net:
             raise UnsupportedRequirement(
                 "Singularity implementation does not support custom networking")
         elif runtimeContext.disable_net: