[ADQL] Fix a bug reported by M. Taylor about qualified table name:

when in a table definition (extension of DBTable) no schema is
specified, it was possible to prefix the table name by a fake
schema name ; it was checked at all:
e.g. "SELECT * FROM no_schema.foo".
This is no longer possible: if no schema is specified, no schema
must be used in an ADQL query. But if one is specified, the schema
prefix is optional.