Fortified op handlers against mising $a

lib/SugarTrail/Template/Condition.pm

 use warnings;
 
 sub _equals {
-    my $a = shift;
+    my $a = shift or return;
     my $b = $_[0]->[0];
     return $a eq $b;
 }
 
 sub _regex {
-    my $a = shift;
+    my $a = shift or return;
     my $b = $_[0]->[0];
     return $a =~ m/$b/;
 }
 
 sub _in {
-    my $a = shift;
+    my $a = shift or return;
     my %b = map { $_ => 1 } @_;
     return exists $b{$a};
 }