The current accepted solution seems to let the dnf resolver decide,
which seems to prefer nano-default-editor, if nano is installed.
The current solution depends on weak dependency system in DNF - default-editor requires
system-default-editor which both - vim-default-editor and nano-default-editor - provides
(*-default-editors conflicts with system-default-editor as well to prevent both being
installed at the same time).
Then if weak-deps are enabled - DNF sees 'Recommends:' in default-editor and
brings in nano-default-editor if there isn't any other *-default-editor already
installed. If there is, the transaction is not done, but it doesn't fail the whole
transaction -> upgrade-path is okay.