fix(pageserver): don't panic if gc-compaction find no keys (#11200)

## Problem

There was a panic on staging that compaction didn't find any keys. This
is possible if all layers selected for compaction does not contain any
keys within the current shard.

## Summary of changes

Make panic an error. In the future, we can try creating an empty image
layer so that GC can clean up those layers. Otherwise, for now, we can
only rely on shard ancestor compaction to remove these data.

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant/timeline/compaction.rs

@@ -3189,7 +3189,11 @@ impl Timeline {
         }
 
         // TODO: move the below part to the loop body
-        let last_key = last_key.expect("no keys produced during compaction");
+        let Some(last_key) = last_key else {
+            return Err(CompactionError::Other(anyhow!(
+                "no keys produced during compaction"
+            )));
+        };
         stat.on_unique_key_visited();
 
         let retention = self