Per-table configuration options are inherited from a hierarchy of configuration scopes. In some cases, it would be useful to denote a key as "do not inherit from the parent" (treat it as though it doesn't exist, possibly falling back on the default if the property is defined in the default configuration).
In such cases, a property key could be prefixed with a "!" character (as in "NOT") to "unset" a key that is set with a value in a parent scope.
This ticket removes the previously undocumented restriction that empty string is the same as a property not being set at all.