Empty properties loaded from config should be considered null

[bbottema]

When a property is empty in the config is empty but not null, currently this value is being considered, but not parsed to the proper type. This could result in ClassCast exceptions if the target type is not a String. Since empty strings are never meaningful in Simple Java Mail, this should be considered as a null-value.

The result then is either the default value passed as extra argument or null.

[bbottema]

Released in 4.1.3.