storage controller: fix handing of tenants with no timelines during scheduling optimization (#7673)

## Problem

Storage controller was using a zero layer count in SecondaryProgress as
a proxy for "not initialized". However, in tenants with zero timelines
(a legitimate state), the layer count remains zero forever.

This caused https://github.com/neondatabase/neon/pull/7583 to
destabilize the storage controller scale test, which creates lots of
tenants, some of which don't get any timelines.

## Summary of changes

- Use a None mtime instead of zero layer count to determine if a
SecondaryProgress should be ignored.
- Adjust the test to use a shorter heatmap upload period to let it
proceed faster while waiting for scheduling optimizations to complete.

test_runner/performance/test_storage_controller_scale.py

@@ -102,6 +102,9 @@ def test_storage_controller_many_tenants(
                 tenant_id,
                 shard_count,
                 stripe_size,
+                # Upload heatmaps fast, so that secondary downloads happen promptly, enabling
+                # the controller's optimization migrations to proceed promptly.
+                tenant_config={"heatmap_period": "10s"},
                 placement_policy={"Attached": 1},
             )
             futs.append(f)