fix: launch_cmd not set when provided to FluxExecutor

Problem: Currently, the launch_cmd is not set if provided to the
FluxExecutor directly. Many jobs are likely to start
already with access to a flux instance, in which case
the launch command should use flux submit instead of
flux start.
Solution: Ensure the launch_cmd is set.

Signed-off-by: vsoch <vsoch@users.noreply.github.com>

parsl/executors/flux/executor.py

@@ -201,8 +201,8 @@ def __init__(
         self.flux_path = os.path.abspath(flux_path)
         self._task_id_counter = itertools.count()
         self._socket = zmq.Context().socket(zmq.REP)
-        if launch_cmd is None:
-            self.launch_cmd = self.DEFAULT_LAUNCH_CMD
+        # Assumes a launch command cannot be None or empty
+        self.launch_cmd = launch_cmd or self.DEFAULT_LAUNCH_CMD
         self._submission_queue: queue.Queue = queue.Queue()
         self._stop_event = threading.Event()
         # lock to protect self._task_id_counter and also submission/shutdown race