Too many empty :root in output when passing variables

[pfrankov]

Here is an issue:
There is a test fixture called test/fixtures/js-defined-preserve-injected.css

.box1 {
	width: var(--js-defined1);
	height: var(--js-defined2);
	background: var(--js-defined-no-prefix);
}

With an output in test/fixtures/js-defined-preserve-injected.expected.css

:root {
}
:root {
}
:root {
}
:root {
}

.box1 {
	width: 75px;
	width: var(--js-defined1);
	height: 80px;
	height: var(--js-defined2);
	background: #ff0000;
	background: var(--js-defined-no-prefix);
}

I'd say that these duplicating :roots are not very useful for any purpose and must be removed from the output.

What do you think?

[MadLittleMods]

Related to #89

It would be nice if they were not in the output. PR welcome to fix it.