Fix panic when generating color fallbacks for gradients using currentColor

[chuganzy]

This PR aims to fix a crash when generating color fallbacks for CSS such as:

.foo {
  background-image:
    linear-gradient(oklch(0% 0 0), oklch(100% 0 0)),
    linear-gradient(currentcolor, currentcolor);
}

When targeting a browser without oklch() support, Lightning CSS generates fallbacks for every gradient in the declaration. This caused it to attempt converting currentColor to RGB, P3, or Lab and panic.

Found the same issue in text-shadow. Making a separate PR for that.