fix: use empty go.sum for go-doc workflow

This prevents actions/setup-go@v4 warning due to missing go.sum.  We don't have any actual dependencies here, we're just forcing pull of this repo through the goproxy to trigger pk.go.dev doc update

.github/workflows/main.yml

@@ -10,6 +10,7 @@ jobs:
   go-docs:
     runs-on: ubuntu-22.04
     steps:
+      - run: touch go.sum
       - uses: actions/setup-go@v4
         with:
           go-version: '1.21'