Add `theme_mods_{$stylesheet}` option during `populate_options()`

[dlh]

After switch_theme(), a theme_mods_{$stylesheet} option is created if one doesn't exist to avoid extra database queries (if I understand #14828 correctly).

No similar option is currently created during installation, so a fresh install can includes those queries if the default theme looks for the mods before the option is added.

The attached patch would add the option during populate_options().

I included a unit test, although I'm not sure where the best location for it would be.

[joyously]

Hmm, my theme actually uses the fact that it is not populated to supply default values and know that it is brand new.