IE11 Crash on call to getPropertyValue when passed undefined as argument

[benflodge]

** Description **
In IE11 the getPropertyValue implementation throws an error on property = property.trim(); https://github.com/nuxodin/ie11CustomProperties/blob/master/ie11CustomProperties.js#L518 when property value is undefined.

In IE11 getPropertyValue should return an empty string if called with undefined as first argument.

** Version **
v4.1.0

** To Reproduce **
https://jsbin.com/bivajug/edit?html,js,console