Allow empty go-whosonfirst-index -mode flag

[thisisaaronland]

Principally for use with the SQLite-related package where the assumption is the -spatial-database-uri flag may reference a database that has already been indexed.

At present the work-around is do this:

go run -mod vendor cmd/server/main.go -... -mode directory://

Where we set the -mode flag but don't pass any files to index.