Update main_py_to_bitly.yml

Author: skysign

.github/workflows/main_py_to_bitly.yml

@@ -11,7 +11,14 @@
           with:
             fetch-depth: 2
         - name: get path of main.py
-          run: echo "path_of_main_py=$(git diff --name-only HEAD~1 HEAD | grep '/main\.py')" >> $GITHUB_ENV
+          run: |
+            echo "zero_length_string="" >> $GITHUB_ENV
+            echo "path_of_main_py=$(git diff --name-only HEAD~1 HEAD | grep '/main\.py')" >> $GITHUB_ENV
+        - name: Existing path of main.py
+          if: ${{ env.zero_length_string }} == ${{ env.path_of_main_py }}
+          run: |
+            echo "No main.py"
+            exit 1
         - name: Decorate path_of_main_py
           run: echo "path_of_main_py=https://github.com/skysign/WSAPT/${{ env.path_of_main_py }}" >> $GITHUB_ENV
         - name: Print path_of_main_py