Ignore :undefined_variable "reason" in getvar

`catch` returns value of second argument to `throw`, which until 860a2761f334c964068038b3ef6853f08beb1df5 was `nil`, but now is non-falsey reason for error. short-circuit using return and eval to nil if `throw` was caught.

lib/puppet/parser/functions/getvar.rb

@@ -21,8 +21,10 @@ module Puppet::Parser::Functions
 
     begin
       catch(:undefined_variable) do
-        self.lookupvar("#{args[0]}")
+        return self.lookupvar("#{args[0]}")
       end
+
+      nil # throw was caught
     rescue Puppet::ParseError # Eat the exception if strict_variables = true is set
     end