flux-resource: fix performance issue generting queue output

Problem: In commit 4243895 a test
for an empty resource set was added to the queue property of the
ResourceSetExtra class to avoid generating a queue for an empty
resource set. However, the test for empty uses self.rlist, which is an
expensive call. Also, the test is done for each queue in the config,
even though the contents of the resource set won't change.

Use self.ranks instead of self.rlist to check for an empty resource
set and move the call out of the loop so it is made only once.

Fixes #5857

Author: grondo

src/cmd/flux-resource.py

@@ -443,10 +443,10 @@ def propertiesx(self):
     def queue(self):
         queues = ""
         if self.flux_config and "queues" in self.flux_config:
+            if not self.ranks:
+                return ""
             properties = json.loads(self.get_properties())
             for key, value in self.flux_config["queues"].items():
-                if not self.rlist:
-                    continue
                 if "requires" not in value or set(value["requires"]).issubset(
                     set(properties)
                 ):