fix: do not mutate non-css strings

[43081j]

Basically, if you have a source file with no CSS, like so:

html`<div></div>`;

it appears postcss will stringify it but have no reference node. in these cases, i think (or hope) we can safely assume we're not stringifying CSS and to just leave it alone.

this was causing backticks in JS to be escaped before.

cc @abdonrd

[coveralls]

Pull Request Test Coverage Report for Build 1671527775

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 95.904%

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Totals
Change from base Build 1602839425:	0.03%
Covered Lines:	635
Relevant Lines:	643

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💛 - Coveralls

[abdonrd]

Yay! Thanks James!