Determining size in EpsImagePlugin

[radarhere]

Suggestion for python-pillow#6879

Reverts a change that seems unnecessary. I don't think _size is ever None apart from this, so it would be better not to change the type of it if we don't have to.

[Yay295]

_size can be None if the BoundingBox header exists but is invalid, and the ImageData comment doesn't exist.

With your change, if the BoundingBox header is invalid but ImageData exists, _size would be set but box wouldn't.

Setting _size to tuple() at the start instead of None works if you want to keep the data type the same.

[Yay295]

I've added a test for that situation: c71fff2