Try to run workflow on pull request.

Codecov has to be skipped because the secret isn't available.

codecov.io understands that this is an issue which needs a better
resolution: codecov/codecov-action#44

.github/workflows/drawdown.yml

@@ -1,6 +1,6 @@
 name: Drawdown Solutions Python application
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
@@ -27,3 +27,4 @@ jobs:
       with:
         token: ${{secrets.CODECOV_TOKEN}}
         file: ./coverage.xml
+      if: secrets.CODECOV_TOKEN != ''