Fix missing space when AtRule#params is set after parsing - #2113

Setting AtRule#params on an at-rule that was parsed without params glues the params onto the name:

let root = postcss.parse('@layer{a{color:black}}')
root.first.params = 'utilities'
root.toString() //=> '@layerutilities{a{color:black}}'

@layerutilities is a single at-word, so the output no longer parses back to the at-rule that produced it. Same for @media{}, @scope{}, @font-face{} — anything written without params.

atruleStart() already has a fallback that inserts the space when raws.afterName is missing, but the parser sets raws.afterName = '' explicitly for a params-less at-rule, so the fallback never gets a chance to run.

I still prefer the raw when it's there, and only fall back to a space when the params would otherwise merge into the at-rule name token. @media(min-width:0) and @import"a.css" stay byte-identical: ( and " end the at-word on their own, and those delimiters are the only way the parser can produce an empty afterName together with non-empty params.

Found this fuzzing mutation round-trips over the postcss-parser-tests fixtures — across ~10k parse → set params → re-parse cases this was the only invariant that broke (100 distinct failures before, 0 after). Added tests for both directions; pnpm test is green.