[css-sizing-3] Define min-content size of replaced elements

[fantasai]

We more or less define the max-content size of replaced elements in https://drafts.csswg.org/css-sizing-3/#intrinsic-sizes but we don't define the min-content size. For most replaced elements this is effectively defined in CSS2 (as either the natural size or 300x150), but for replaced elements with an aspect ratio but not a natural size we should probably define it as zero.