Entering Like=* in the symbol column works from some basic testing but I'm not checking against blank fields. To the second question see this idea:
Nope, no filtering works for me on Symbol (CL) column. Sorting works.
Other columns work with '=' and exact match, and like with wildcards '*' or '%'
In the linked question D8033, it was suggested to use '%' wildcard, but it doesn't work with '=', just with like.