update ASTFilter to filter nil results
update ASTFilter to filter nil results

additionally improved performance by using hash value dereference (i.e.
@types[]) to filter AST types

surprisingly Hash[] beats out using key? or a Set. Benchmark using IPS
is here:

https://gist.github.com/abargnesi/7e6339e15e04c0a6e6890c35fc702c47

nbargnesi/bel-parser,nbargnesi/bel-parser,OpenBEL/bel_parser,OpenBEL/bel_parser,OpenBEL/bel_parser,nbargnesi/bel-parser