Update typing

Author: AdrienVannson

src/betterproto/plugin/models.py

@@ -600,8 +600,8 @@ def pydantic_imports(self) -> Set[str]:
 
 @dataclass
 class MapEntryCompiler(FieldCompiler):
-    py_k_type: Type = None
-    py_v_type: Type = None
+    py_k_type: Optional[Type] = None
+    py_v_type: Optional[Type] = None
     proto_k_type: str = ""
     proto_v_type: str = ""