Return null instead of current date

[KerVitarLaeda]

Perhaps should add a 'nothing' checks

if (isUndefinedOrNull(value)) return value;

in amUtcOffset and amTimezone filters
Just like it has already done in other filters (amCalendar, amDifference, etc).

[djuerges]

+1 for this issue - when having undefined values, it is nevertheless rendered as current date, which is unexpected and certainly hard to figure out as well