Get len(candidates) and branch based on that

Cython will turn this into a very efficient `switch...case` statement.
So this cuts down on the overhead of comparisons and checks (paying them
once when computing the length). Then focuses on just checking this C
typed integral value for which `case` to run.

distributed/scheduler.py

@@ -6892,9 +6892,10 @@ def decide_worker(
                 else:
                     return None
 
-    if not candidates:
+    ncandidates: Py_ssize_t = len(candidates)
+    if ncandidates == 0:
         ws = None
-    elif len(candidates) == 1:
+    elif ncandidates == 1:
         for ws in candidates:
             break
     else: