Check for secret key before nightly builds (#1375)

.github/workflows/docker-nightly.yaml

@@ -10,8 +10,14 @@ jobs:
     steps:
 
       - name: Log into GitHub Container Registry
+        env: 
+          GH_CR_PAT: ${{ secrets.GH_CR_PAT }}
         run: echo "${{ secrets.GH_CR_PAT }}" | docker login https://ghcr.io -u ${{ github.actor }} --password-stdin
+        if: env.GH_CR_PAT != null
 
       - uses: actions/checkout@v2
       - name: Setup and run
+        env: 
+          GH_CR_PAT: ${{ secrets.GH_CR_PAT }}
         run: cd build/release && ./docker-nightly.sh
+        if: env.GH_CR_PAT != null