Defining variable in a mixin fails and crashes

[gjhltn]

Defining a variable in a mixin crashes when forEach is called on an undefined variabale.

> Cannot read properties of undefined (reading 'forEach')

To reproduce: in the playground at https://madlittlemods.github.io/postcss-css-variables/playground/

@define-mixin foo {
	--line-height: 100px;
      line-height: var(--line-height);
}