Return an empty bytestring from tobytes() for an empty image

[radarhere]

Resolves #5931

>>> from PIL import Image
>>> Image.new("RGB", (1, 2)).tobytes()
b'\x00\x00\x00\x00\x00\x00'
>>> Image.new("RGB", (1, 1)).tobytes()
b'\x00\x00\x00'

given this progression, you would expect

>>> Image.new("RGB", (1, 0)).tobytes()

to give

b''

However, instead it currently throws an error.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 748, in tobytes
    e.setimage(self.im)
SystemError: tile cannot extend outside image

This PR returns an empty bytestring instead.