fix(cli): Python blank app should call app.synth(), not app.run()

Fixes #3123

Author: garnaat

Diff for: packages/aws-cdk/lib/init-templates/app/python/app.template.py

@@ -8,4 +8,4 @@
 app = core.App()
 %name.PascalCased%Stack(app, "%name%-cdk-1")
 
-app.run()
+app.synth()