Fix misleading error when db.url is undefined

[dcousens]

When config.db.url was undefined - although not allowed by our types - users were faced with the following error:

TypeError: Cannot read properties of undefined (reading 'startsWith')

This can happen if, for example, someone used db.url: process.env.DATABASE_URL!.
You shouldn't do this, but, the error doesn't need to be terrible, when the alternative is that Prisma will inform you that your connection url is empty.

Some day, we should use something like zod to validate a users configuration.

[codesandbox-ci]

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