CLI UX: Nicer output for `cdktf synth` if there are no stacks in an app

[ansgarm]

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Description

Currently running cdktf synth on an app with no stack will output the following:

> cdktf synth

Generated Terraform code for the stacks:

We sould show a different text (including some help for example) if no stacks were found

References

[kination]

@ansgarm is just no stack exists will be fine?