Add set_overlay to NullPreview

It does nothing, of course, we just want it to have the same interface
as all the other preview implementations.

Signed-off-by: David Plowman <[email protected]>

Author: davidplowman

picamera2/previews/null_preview.py

@@ -32,6 +32,10 @@ def start(self, picam2):
         self.thread.start()
         self.event.wait()
 
+    def set_overlay(self, overlay):
+        # This only exists so as to have the same interface as other preview windows.
+        pass
+
     def handle_request(self, picam2):
         completed_request = picam2.process_requests()
         if completed_request: