Spine drop batches once physical compaction frontier is empty

Once the physical compaction frontier is empty, there are no times
that can be represented in a a trace, which means that we're good to
drop all contents.

Signed-off-by: Moritz Hoffmann <antiguru@gmail.com>

src/trace/implementations/spine_fueled.rs

@@ -482,6 +482,10 @@ where
             }
         }
 
+        if self.physical_frontier.is_empty() {
+            self.drop_batches();
+        }
+
         // Having performed all of our work, if more than one batch remains reschedule ourself.
         if !self.reduced() {
             if let Some(activator) = &self.activator {