Fix code scanning alert - Log Injection

[sleberknight]

Tracking issue for:

• https://github.com/kiwiproject/kiwi/security/code-scanning/15

Instead of logging the object, instead log the type of object it is (and handle null appropriately, possibly by requiring the argument be not null). This way the log messages would look like:

"Unable to get property value startDate from object of type Employee"

where startDate is the field name and Employee is the type.