Create a single value cache for the () query key
Since queries using `()` as the key can only store a single value, specialize for that case.
This looks like a minor performance improvement:
<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>Before</b></th><td colspan="2"><b>After</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">1.8477s</td><td align="right">1.8415s</td><td align="right"> -0.33%</td></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.2666s</td><td align="right">0.2655s</td><td align="right"> -0.40%</td></tr><tr><td>🟣 <b>syntex_syntax</b>:check</td><td align="right">6.3943s</td><td align="right">6.3686s</td><td align="right"> -0.40%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">1.6413s</td><td align="right">1.6345s</td><td align="right"> -0.42%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">1.0337s</td><td align="right">1.0313s</td><td align="right"> -0.24%</td></tr><tr><td>Total</td><td align="right">11.1836s</td><td align="right">11.1414s</td><td align="right"> -0.38%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9964s</td><td align="right"> -0.36%</td></tr></table>
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