[Benchmarks] fix running benchmarks when umf dir is not specified

benchmarks += s.benchmarks() was failing since benchmarks returned None

Author: igchor

scripts/benchmarks/benches/umf.py

@@ -30,7 +30,7 @@ def setup(self):
 
     def benchmarks(self) -> list[Benchmark]:
         if not isUMFAvailable():
-            return
+            return []
 
         benches = [
             GBench(self),