fix: remove empty-string PM from CI matrix

pm: "" (auto-detect) fails because we strip packageManager from
package.json in the test job, leaving nothing to detect. pnpm and
yarn variants cover the needed PM testing.

Author: thejackshelton

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
         experimental: [false]
         runtime: [node, deno, bun]
-        pm: ["", pnpm, yarn]
+        pm: [pnpm, yarn]
 
     steps:
       - uses: actions/checkout@v4