pageserver: remove shard split hard link assertion (#9829)

## Problem

We were hitting this assertion in debug mode tests sometimes.

This case was being hit when the parent shard has no resident layers.
For instance, this is the case on split retry where the previous attempt
shut-down the parent and deleted local state for it. If the logical size
calculation does not download some layers before we get to the
hardlinking, then the assertion is hit.

## Summary of Changes

Remove the assertion. It's fine for the ancestor to not have any
resident layers at the time of the split.

Closes https://github.com/neondatabase/neon/issues/9412

pageserver/src/tenant/mgr.rs

@@ -1719,10 +1719,11 @@ impl TenantManager {
                     parent_layers.push(relative_path.to_owned());
                 }
             }
-            debug_assert!(
-                !parent_layers.is_empty(),
-                "shutdown cannot empty the layermap"
-            );
+
+            if parent_layers.is_empty() {
+                tracing::info!("Ancestor shard has no resident layer to hard link");
+            }
+
             (parent_timelines, parent_layers)
         };