Fix default value

See https://github.com/orgs/community/discussions/26322

Author: BenjaminRodenberg

.github/workflows/run-tutorials.yml

@@ -19,7 +19,7 @@ jobs:
   run_ht_simple:
     name: Run HT, simple
     runs-on: ubuntu-latest
-    container: precice/precice:${{ inputs.docker_image_tag }}
+    container: precice/precice:${{ inputs.docker_image_tag || 'nightly' }}
     steps:
       - name: Checkout Repository
         uses: actions/checkout@v2
@@ -55,7 +55,7 @@ jobs:
   run_ht_complex:
     name: Run HT, complex
     runs-on: ubuntu-latest
-    container: precice/precice:${{ inputs.docker_image_tag }}
+    container: precice/precice:${{ inputs.docker_image_tag || 'nightly' }}
     steps:
       - name: Checkout Repository
         uses: actions/checkout@v2