remove empty value declaration from style tree

[tanhauhau]

Fix #3801
If the value is empty, will cause compile error down the road.
Remove it upfront, will be able to treat it as empty rule, if there's no other declaration for the rule.

[Rich-Harris]

I merged this but then reverted it — I think we should probably be throwing a deliberate error here

[tanhauhau]

But it's a valid syntax to the css-tree/parser?