imcc: never store STRINGNULL in registers

performance-wise it would be a good idea, but the vm is not prepared
to deal with it. e.g. index fails, lots of invalid encoding errors.
maybe we should set the empty string to ascii.
[imcc] empty named constants can only be subs

on the original code this looked like
(strncmp(unquoted_name, "Sub",       name_length) with
name_length = 0, for .const  $Pxx = "constant"