test: allow no vectored gets happening (#7939)

when running the regress tests locally without any environment variables
we use on CI, `test_pageserver_compaction_smoke` fails with division by
zero. fix it temporarily by allowing no vectored read happening. to be
cleaned when vectored get validation gets removed and the default value
can be changed.

Cc: #7381

test_runner/regress/test_compaction.py

@@ -85,7 +85,13 @@ def test_pageserver_compaction_smoke(neon_env_builder: NeonEnvBuilder):
 
     vectored_sum = metrics.query_one("pageserver_layers_visited_per_vectored_read_global_sum")
     vectored_count = metrics.query_one("pageserver_layers_visited_per_vectored_read_global_count")
-    vectored_average = vectored_sum.value / vectored_count.value
+    if vectored_count.value > 0:
+        assert vectored_sum.value > 0
+        vectored_average = vectored_sum.value / vectored_count.value
+    else:
+        # special case: running local tests with default legacy configuration
+        assert vectored_sum.value == 0
+        vectored_average = 0
 
     log.info(f"{non_vectored_average=} {vectored_average=}")