Navigation: Delegate the back navigation to the OS if there is no screen to navigate back to.

[CodeRoutineDev]

This is a fix for the issue #179 which I immediately noticed after installing the app.

Since this is such a small PR, I figured simply opening it makes discussion easier than discussing any changes up front. Let me know what you think.

Heads up for the great work on this app. I am really excited about it.

[CodeRoutineDev]

Oh yeah, and this is my first PR on GitHub ever, so let me know if something is missing.

[pynicolas]

Thanks for opening this PR!
Can you please describe the concrete problem that you're trying to fix?
If it's about the arrow icon at the top-left of the camera screen, an issue (#179) was created yesterday because this button doesn't trigger anything (this problem was introduced in version 1.21.0): I'm going to remove this button.
I believe that a system "back" (like a "back" gesture) works correctly: on the camera screen, it brings back to the OS.

[pynicolas]

Sorry, I read the description of your PR too fast. So, yes, you're trying to fix #179.
And as I wrote, I prefer to not display the arrow icon when it should bring the user back to the OS.
I created #181 for that.
Thanks again for your PR!