Fix Rule#selectors losing the empty selector - #2129

8.5.25 made list.split() return [] for an empty string (#2121), so list.comma('') went from [''] to [], and with it Rule#selectors on a rule whose selector is empty.

Plugins read that getter and assume at least one entry. postcss-nested's mergeSelectors() loops over parent.selectors and joins the result back into the child, so an empty parent selector now wipes the child's:

postcss([nested]).process('{ color: red; .dark & { color: blue } }', { from: undefined }).css
// 8.5.24: '{ color: red; } .dark  { color: blue }'
// 8.5.25: '{ color: red; }  { color: blue }'

That is what the Panda CSS reproduction in #2128 hits: the selector is wiped, the block is merged away, and .dark is gone from the tokens layer.

'' is a string, and its last: true split has always been [''], same as the trailing item in list.comma('a, b,'). So this narrows the guard to non-strings, which keeps what #2121 was after (list.split(undefined) no longer throws) without changing the empty-string result.

Fixes #2128