Fix Buffer.mapAsync

Recent optimizations skip waiting for a return value if the Function was
annotated with None as return type. In Buffer.mapAsync we still need to wait
for the mapping to be complete, thus change the return value to some
  Dummy class type.

Author: mhochsteger

webgpu/webgpu_api.py

@@ -1101,8 +1101,14 @@ async def requestDevice(
         return Device(device)
 
 
+class _DummyNone:
+    pass
+
+
 class Buffer(BaseWebGPUHandle):
-    async def mapAsync(self, mode: MapMode, offset: int = 0, size: int = 0) -> None:
+    async def mapAsync(
+        self, mode: MapMode, offset: int = 0, size: int = 0
+    ) -> _DummyNone:  # don't return None, as this would ignore the blocking wait
         return await self.handle.mapAsync(mode, offset, size)
 
     def getMappedRange(self, offset: int = 0, size: int = 0) -> int: