Don't attempt to push to nuget.org without a key

Author: kzu

.github/workflows/publish.yml

@@ -40,4 +40,7 @@ jobs:
           path: '*.binlog'
 
       - name: 🚀 nuget
+        env:
+          NUGET_API_KEY: ${{ secrets.NUGET_API_KEY }}
+        if: env.NUGET_API_KEY != ''
         run: dotnet nuget push ./bin/**/*.nupkg -s https://api.nuget.org/v3/index.json -k ${{secrets.NUGET_API_KEY}} --skip-duplicate