ensure requirements.txt not zero length

Author: gilesknap

.github/workflows/code.yml

@@ -95,9 +95,11 @@ jobs:
           touch requirements.txt 
           touch requirements_dev.txt
           pip install -r requirements.txt -e .
-          pip freeze --exclude-editable >> requirements.txt
+          pip freeze --exclude-editable > requirements.txt
           pip install -r requirements_dev.txt -e .[dev]
-          pip freeze --exclude-editable >> requirements_dev.txt
+          pip freeze --exclude-editable > requirements_dev.txt
+          # ensure non-zero length requirements.txt
+          echo "# runtime dependencies" >> requirements.txt
 
       - name: Install with latest dependencies
         if: ${{ ! matrix.lock }}