fixed fixed_string expression eval for go

[bright-starry-sky]

fixed bug for expression eval when prop data type is fixed_string
Repetition steps：

CREATE SPACE test_space (partition_num=1,replica_factor=1, vid_type=fixed_string(30));
USE test_space
create tag t1(c1 fixed_string(40))
create edge e1(c1 fixed_string(40))
insert vertex t1(c1) values "1":("row"), "2":("row"), "3":("row")
insert edge e1(c1) values "1" -> "2"@0:("row"), "1" -> "3"@0:("row")
go from "1" over e1 where e1.c1 == "row"
go from "1" over e1 where $$.t1.c1 == "row"
go from "1" over e1 where $^.t1.c1 == "row"

[critical27]

LGTM