Prevented unnecessary warning

lib/SugarTrail/Template/Slave.pm

@@ -40,7 +40,7 @@ sub warnings {
     foreach my $step (@{$self->{steps}}) {
         push @warnings, $_ foreach @{$step->{warnings}};
     }
-    my $warnings;
+    my $warnings = "";
     if (@warnings) {
         $warnings .= "---\n# SUGAR-TRAIL WARNINGS #\n\n";
         foreach (@warnings) {