…se UnknownAttribute exception (mongodb#5836)

* database_field_name given nil or empty string should raise UnknownAttribute exception

* fix spec syntax

* database_field_name return empty string instead of exception

* `field` might be an empty string, not merely nil

* fix specs to expect empty string instead of nil

This is okay, because the database_field_name method is a private API.
We can change the contract here without regard for who else might
be using it.

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Co-authored-by: Jamis Buck <[email protected]>