Initial migration with CLI can't infer directory

[ignaciojcano]

Just set up following the example of sequelize-typescript, but when creating the first migration using the CLI (just like in the example), I kept getting Error: Couldn't infer a directory to generate migration file in. Pass --folder explicitly. If you pass in the folder directly it works, but kinda expected it to infer it from the migrations configuration.

Currently if --folder is not passed in, it will try to grab the last migration that exists and use that one's dirname (https://github.com/sequelize/umzug/blob/master/src/umzug.ts#L393)

Two options come to mind:

1. Add to the error that it couldn't be inferred because there are no existing migrations.
2. Change how we infer the directory of the migrations

Anyways, its pretty mild, everything else so far has been 👍