this patch will give the 'undefined' variables in NEdit
macros a Perl-like behavior, which returns 0 (zero)
when evaluated as integer, or returns "" (empty string)
when evaluated as string.
This way we no longer need to pre-define the variables
in ~/.neditmacro before using them.
Use isnull() to check if a variable is undefined
(null), if you want to. To undef a variable, simply
assign an unused var name to it, probably some uncommon
var name like UNDEF to avoid confusion.