Check camera is be configured in start_encoder()

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>

Author: davidplowman

picamera2/picamera2.py

@@ -2368,6 +2368,8 @@ def start_encoder(self, encoder=None, output=None, pts=None, quality=None, name=
                 output = FileOutput(output, pts=pts)
             _encoder.output = output
         streams = self.camera_configuration()
+        if not streams:
+            raise RuntimeError("Camera has not been configured")
         if name is None:
             name = self.encode_stream_name
         if streams.get(name, None) is None: