build-export: Proper validation of Exec line in desktop file

[dbrgn]

Here's an attempt at explicitly allowing empty Exec values, as discussed here: flatpak/flatpak-docs#324 (comment)

Additionally, I found and fixed a segfault that occurred when the Exec line is omitted from the desktop file.

[mwleeds]

It's a little strange that Flatpak allows build-export to succeed with warnings even when the desktop file is invalid. I assume the reasoning is that the warnings (those generated in validate_desktop_file() or those generated by desktop-file-validate) might not be critical errors so the app may still be usable. But desktop-file-validate only returns a non-zero status when the file is breaking a spec requirement, not when there's merely a warning. desktop-file-validate doesn't currently return a non-zero status when the Exec= key is missing and DBusActivatable=true is not set, but it should.

That all makes me wonder if we should be allowing the export of desktop files that have no Exec= line and don't have DBusActivatable=true.

In any case, I added a commit here to avoid printing a warning when Exec= is missing but DBusActivatable=true is present, so that will need to be reviewed by another maintainer.