Handle incorrect use of inputs

If the user mis-configures the wizard, e.g. using "is" instead of "=" it will produce an exception here in certain circumstances. This prevents that.

lib/custom_wizard/mapper.rb

@@ -137,8 +137,10 @@ class CustomWizard::Mapper
       elsif ["true", "false"].include?(value)
         result = key.public_send(operator, ActiveRecord::Type::Boolean.new.cast(value))
       end
-    else
+    elsif [key, value, operator].all? { |i| !i.nil? }
       result = key.public_send(operator, value)
+    else
+      result = false
     end
     
     if operator == '=~'