Don't attempt to rename table if it doesn't exist

[dodget]

Thanks for this package, it was a big help! One thing I noticed while trying to use it is that the following query was tripping up on models/tables that had been removed at some point from the original app:

query = (
    f'ALTER TABLE "{old_table_name}" '
    f'RENAME TO "{new_table_name}"'
)

Adding IF EXISTS to the query helped to skip tables that no longer exist.

query = (
    f'ALTER TABLE IF EXISTS "{old_table_name}" '
    f'RENAME TO "{new_table_name}"'
)

I can fork and submit a PR if you'd like.