[css-masking-1] Disallow trailing `/` in `mask-border`

[cdoublev]

The syntax of border-image and mask-border are very close but for border-image, the CSSWG resolved on disallowing trailing / a long time ago:

I have four proposals to address these:

A. Leave the current syntax as it is.
B. Allow more tailing slashes.
C. Disallow trailing slash at the end.
D. Disallow empty component.

We went with option C, which was the original intent.

I think the reasoning to disallow a trailing / in border-image should also apply to mask-border.