Merge pull request #4672 from ESMCI/jgfouca/fix_append_key

Fix for PES append feature

CIME/case/case.py

@@ -1234,7 +1234,7 @@ def _setup_mach_pes(self, pecount, multi_driver, ninst, machine_name, mpilib):
                     and value.endswith("'")
                 ):
                     value = value[1:-1]
-                if append[key]:
+                if key in append and append[key]:
                     ovalue = self.get_value(key)
 
                 self.set_value(key, value + " " + ovalue)