RSDK-5174: Fix get_image mimetype handler in python sdk

[hexbabe]

https://viam.atlassian.net/browse/RSDK-5174

The first conditional branch of get_image_from_response never gets hit because the default mime_type value in the invoker— get_image in client.py — is "" and not None.

[cheukt]

is there a corresponding test we can add?

[hexbabe]

is there a corresponding test we can add?

I think a test exists for this use case i.e. empty mime type, but we don't invoke the helper function, get_image_from_response, that actually handles the logic. We do it manually like this:

            # Test empty mime type. Empty mime type should default to JPEG for non-depth cameras
            request = GetImageRequest(name="camera")
            response: GetImageResponse = await client.GetImage(request)
            assert service._camera_mime_types["camera"] == CameraMimeType.JPEG

[hexbabe]

I also tested with an actual Python sdk client and it works now no args

[seanavery]

LGTM!