synapse_port_db misbehaves unintuitively if you pass the wrong path to the sqlite.db

[ara4n]

and tries to migrate an empty DB

[DMRobertson]

If anyone wants to take this on:

• A good first step would be to use os.path.isfile just before here to check that args.sqlite_database is a file.
• A more robust approach would be to follow the advice here and try to open the database in read-only mode (r) rather than read-write-create (rwc). (AFAICS the script only ever reads from the sqlite db and writes to the postgres DB.