Fix migrate! macro with no params

[esemeniuc]

Fixes issue experienced in #746 when using no parameter:

error: paths relative to the current file's directory are not currently supported
 --> src/db.rs:7:5
  |
7 |     sqlx::migrate!().run(&conn).await?;
  |     ^^^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

[izik1]

LGTM on CI completion.
For future work ideally we make the ./ prefix redundant, but that's forward compatible so no big deal.
I'll open an issue for that after this is merged