fix(parser): Fixed parsing of empty alternation groups (#193)

[metthal]

This fixes parsing of regexes like /(a|b|)/ which are valid YARA but we weren't able to handle empty alternation groups. Fixes #193.

[MatejKastak]

LGTM 👍

I also tested and it is also accepted by YARA.

rule asd {
    strings:
        $1 = /(a|||||)/ 
    condition:
        $1
}