Migrate: Postgres connection string without port prints `undefined`

I have a Postgres connection string of this form:
```
postgres://postgres:pw@lol.hello.eu-west-1.rds.amazonaws.com/my-db
```

When I do `prisma db push`, I get this:
```
Datasource "db": PostgreSQL database "my-db", schema "public" at "lol.hello.eu-west-1.rds.amazonaws.com:undefined"
```
It seems like we should fill in the default value for the port if it's not in the connection string.