fix: reject empty middle wildcard segments

[mikamikasuki]

Summary

Keep TrieRouter and LinearRouter consistent with RegExpRouter and PatternRouter for middle wildcards. A route such as /a/*/b should require a non-empty segment, but currently matches /a//b in TrieRouter and LinearRouter.

This change rejects empty middle wildcard segments and preserves trailing wildcard behavior.

Validation

• pnpm run build
• pnpm run check (0 errors; existing warnings)
• env -u NO_COLOR FORCE_COLOR=1 pnpm run test (5,208 passed, 44 skipped)
• The shared router regression failed before the fix in TrieRouter and LinearRouter, then passed for all four routers.

The author should do the following, if applicable

• Add tests
• Run tests
• pnpm run check:fix to format and lint the code
• Add TSDoc/JSDoc to document the code