fix: stop force-updating css variables

[innocenzi]

This pull request fixes a case where the CSS variable would be added as a style on the selected element, hence making useless the observation of said element in cases where the style is not changed but a class or an attribute is.

This version will behave as before, adding a style since we provide a value:

useCssVariables({
  onBackgroundColor: {
    name: "color-on-background",
    value: "red" // we define a value, so style is applied
  }
});

This version will behave as expected in the first PR, not adding a style if not required:

useCssVariables({
  onBackgroundColor: 'on-background-color' // we don't define a value, no style is applied
});

[pikax]

Thank you 👍