Undefined array key "map_false" ConvertBoolean.php:47

Problem/Motivation

When running this in a migration using the convert_boolean plugin, we receive this error message. We also receive one with "map_true"

Steps to reproduce

Create a boolean field on a node and import a string value of "TRUE" onto this field.

Proposed resolution

I believe the issue lies on lines 47 and 51 for accessing optional map_false and map_true config values. There should be a check to ensure those keys exist on $this->configuration before trying to access them.