fix error when testmods not defined

Author: jedwards4b

scripts/lib/CIME/test_scheduler.py

@@ -801,7 +801,7 @@ def _run_phase(self, test):
             return True, "SKIPPED"        
         else:
             # SmartSim test needs a wrapper to submit
-            if "drv/smartsim" in testmods:
+            if testmods and "drv/smartsim" in testmods:
                 cmd = os.path.join(self._cime_root, "tools", "smartsim", "launch.py")
             else:
                 cmd = "./case.submit"