a #GdaConnection, or null
a #GdaServerProvider, or null
a complete path to a node (starting with "/")
a new string, or null if the value is undefined or if the path is not defined or path does not hold any value, or if the value held is not a string (in that last case a warning is shown).
This method is similar to gda.server_operation.ServerOperation.getValueAt, but for SQL identifiers: a new string is returned instead of a #GValue. Also the returned string is assumed to represents an SQL identifier and will correctly be quoted to be used with cnc, or prov if cnc is null (a generic quoting rule will be applied if both are null).