fill(none) fails on a grayscale image #421
Description
When I try to apply a fill(none) filter to a grayscale image (attached), I receive the following error:
{"message":"linear: vector must have 1 or 2 elements","status":406}
Could be reproduced easily using the URL below, which uses the image attached to this issue (assuming that imagor is running locally on port 8000):
http://localhost:8000/unsafe/fit-in/200x200/filters:fill(none)/https://github.com/cshum/imagor/assets/487261/88429e96-4bae-4ebb-a7a5-005e070319f6
