Call on void target in {DS_SPARSE_TABLE_KEYS}.twin

By calling `twin' on the keys of a hash table, following exception occurs:


-------------------------------------------------------------------------------
Class / Object      Routine                Nature of exception           Effect
-------------------------------------------------------------------------------
DS_SPARSE_TABLE_KEYS
                    copy @3                copy:
<00007F271CD4F598>                         Feature call on void target.  Fail
-------------------------------------------------------------------------------
DS_SPARSE_TABLE_KEYS
                    copy @3
<00007F271CD4F598>                         Routine failure.              Fail
-------------------------------------------------------------------------------
DS_SPARSE_TABLE_KEYS
                    twin @3
<00007F271CD4F508>  (From ANY)             Routine failure.              Fail
------------------------------------------------------------------------------


To reproduce call following feature


	test_ds_sparse_table_keys
		local
			l_table: DS_HASH_TABLE [STRING, STRING]
			l_list: DS_BILINEAR [STRING]
		do
			create l_table.make_default
			l_list := l_table.keys.twin
		end

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