`WebPDecoder::into_frames` returns an empty iterator for non-animated images

[SludgePhD]

image 0.25.9

Expected

I would expect the iterator to yield precisely one frame, since the image contains one frame.

Alternatively, yielding an error ("image is not animated") might also make sense.

Actual behaviour

The operation silently fails and produces zero frames.

Reproduction steps

Load a non-animated WebP with the WebPDecoder. Then call WebPDecoder::into_frames().collect_frames()?.