fts: Fix search query generation when tokens are filtered out.

For example with English stopwords when searching "foo and bar" it should
be equilevant to "foo bar", since "and" is filtered out.

src/plugins/fts/fts-search-args.c

@@ -81,6 +81,11 @@ fts_backend_dovecot_expand_tokens(struct fts_filter *filter,
 		} else if (ret < 0) {
 			*error_r = t_strdup_printf("Couldn't filter search token: %s", error);
 			return -1;
+		} else {
+			/* The filter dropped the token, which means it was
+			   never even indexed. Ignore this word entirely in the
+			   search query. */
+			return 0;
 		}
 	}
 	array_sort(&tokens, i_strcmp_p);