Return nil if image size is zero on iTerm2

[yoichi]

On iTerm2, size 0 image is passed to the png encoder and cause error in it:

$ ./longcat -H  -l 0
2019/10/12 09:44:18 png: invalid format: invalid image size: 0x0

By returning first as with other backends, avoiding Encoder error.

[mattn]

Thank you