Additional whitespace being inserted in `var(--foo,)` and `env(--foo,)`

[RobinMalfait]

I was bumping the Lightning CSS dependencies in Tailwind CSS, and I noticed that our tests started failing. I noticed that if you have a CSS variable with a fallback, then an additional whitespace character is being inserted. If you then run the output through Lightning CSS again then an additional one is being inserted and so on.

I believe it was related to this commit: 32f1f52 or it least it didn't happen in 1.30.2 and it started happening in 1.31.0.

Input:

.foo {
  transform: var(--bar, );
}

Output v1.31.0

.foo {
  transform: var(--bar,  );
}

Output v1.30.2

.foo {
  transform: var(--bar, );
}