Simplify DSN file (#475)

* Add checks for empty sentry DSN

* Add default DSN key

* Fix CI workflows

.github/workflows/android.yaml

@@ -37,5 +37,4 @@ jobs:
     - name: Build for Android
       run: |
         flutter pub get
-        cp lib/dummy_dsn.dart lib/dsn.dart
         flutter build apk --debug

.github/workflows/ci.yaml

@@ -43,7 +43,6 @@ jobs:
           python3 collect_translations.py
       - name: Static Analysis Tests
         run: |
-          cp lib/dummy_dsn.dart lib/dsn.dart
           python3 find_dart_files.py
           flutter pub get
           flutter analyze

.github/workflows/ios.yaml

@@ -38,5 +38,4 @@ jobs:
         pod repo update
         pod install
         cd ..
-        cp lib/dummy_dsn.dart lib/dsn.dart
         flutter build ios --release --no-codesign --no-tree-shake-icons