Fix errors on empty any

Author: AdrienVannson

src/betterproto2_compiler/known_types/any.py

@@ -20,24 +20,34 @@ def pack(self, message: betterproto2.Message, message_pool: "betterproto2.Messag
         self.type_url = message_pool.type_to_url[type(message)]
         self.value = bytes(message)
 
-    def unpack(self, message_pool: "betterproto2.MessagePool | None" = None) -> betterproto2.Message:
+    def unpack(self, message_pool: "betterproto2.MessagePool | None" = None) -> betterproto2.Message | None:
         """
         Return the message packed inside the `Any` object.
 
         The target message type must be registered in the message pool, which is done automatically when the module
         defining the message type is imported.
         """
+        if not self.type_url:
+            return None
+
         message_pool = message_pool or default_message_pool
 
-        message_type = message_pool.url_to_type[self.type_url]
+        try:
+            message_type = message_pool.url_to_type[self.type_url]
+        except KeyError:
+            raise TypeError(f"Can't unpack unregistered type: {self.type_url}")
 
         return message_type().parse(self.value)
 
     def to_dict(self, **kwargs) -> dict[str, typing.Any]:
+        # TODO allow passing a message pool to `to_dict`
         output: dict[str, typing.Any] = {"@type": self.type_url}
 
         value = self.unpack()
 
+        if value is None:
+            return output
+
         if type(value).to_dict == betterproto2.Message.to_dict:
             output.update(value.to_dict(**kwargs))
         else: