Ensure unflap doesn't crash on unknown interface

The submitted interface argument may not have any flap stats, but
there is no need for the implementation to crash if it doesn't.  It's
safe to quietly ignore it, as this means the interface is already
"unflapped".

src/zino/flaps.py

@@ -105,9 +105,9 @@ def first_flap(self, interface: PortIndex) -> FlappingState:
         self.interfaces[interface] = flap
         return flap
 
-    def unflap(self, interface: PortIndex) -> FlappingState:
-        """Removes all flapping stats tracking for a port"""
-        return self.interfaces.pop(interface)
+    def unflap(self, interface: PortIndex) -> Optional[FlappingState]:
+        """Removes all flapping stats tracking for a port, if they exist"""
+        return self.interfaces.pop(interface, None)
 
     def is_flapping(self, interface: PortIndex) -> bool:
         """Returns True if the current stats indicate that the interface has crossed the flapping threshold.