LEFT JOIN
Fixed that in certain situations MATCH ... AGAINST returns false hits
for NULLs produced by LEFT JOIN when there is no fulltext index
available.


mysql-test/r/fulltext_left_join.result:
  A test case for BUG#25729.
mysql-test/t/fulltext_left_join.test:
  A test case for BUG#25729.
sql/item_func.cc:
  concat_ws(NULL) returns empty string instead of NULL. There is not much
  sense to calculate relevance for empty strings, thus return 0 immediately
  if we got NULL or empty string from concat_ws.
sql/item_func.h:
  Item_func_match::concat renamed to concat_ws.