Bug fix for toga.Selection on android for value

[LunaMeadows]

Added an if statement for get_selected_item that checks if the Selection has values or not. If it does not it will return None which is consistent with linux and windows. If it has values, it will return "" with type str if an empty value is selected.

Makes the return value consistent across platforms

Fixes #1723

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the PR. In this case, I think Android may have been the one instance doing it right - the issue is with the other platforms :-) I've pushed an update to the selection example that provides a demonstration case; returning None when there isn't anything to select seems like the right outcome, rather than "" which could be ambiguous with a legitimate (albeit ill-advised) actual list entry with no label.

On that basis, I've fixed the cocoa and iOS implementations as well; if you want to try a winforms and/or GTK fix before I get to it tomorrow morning my time (~12 hours from now), feel free.

[LunaMeadows]

I can take care of the GTK platform. My windows machine is still out of service right now.

[freakboy3742]

Looks like the Winforms and GTK implementations are already doing the right thing, so I think this is good to go! Thanks for doing the initial investigation on this.