FIX: Only remove temporary folder if it is present (#4379)

This was causing an error, when running the tests/engine/test_calc_job.py on OSX,
since here it is not guaranteed the temporary folder will be created.

aiida/engine/processes/calcjobs/calcjob.py

@@ -312,7 +312,8 @@ def parse(self, retrieved_temporary_folder=None):
         try:
             exit_code_retrieved = self.parse_retrieved_output(retrieved_temporary_folder)
         finally:
-            shutil.rmtree(retrieved_temporary_folder, ignore_errors=True)
+            if retrieved_temporary_folder is not None:
+                shutil.rmtree(retrieved_temporary_folder, ignore_errors=True)
 
         if exit_code_retrieved is not None and exit_code_retrieved.status > 0:
             msg = f'output parser returned exit code<{exit_code_retrieved.status}>: {exit_code_retrieved.message}'