fix(serveStatic): add guard to prevent reading empty folders #5913

Job	Run time
Checking if it's valid for JSR	9s
Deno	12s
Bun	5s
Fastly Compute	1m 1s
Lambda@Edge	1m 5s
AWS Lambda	1m 4s
Node.js v18.18.2	1m 9s
Main	2m 4s
Node.js v20.x	1m 1s
perf-measures-type-check-on-main	0s
perf-measures-type-check-on-pr	16s
Node.js v22.x	1m 2s
workerd	1m 5s
Coverage	9s
	10m 22s

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