Don't allocate unnecessary annotation arrays in IonValueLite's copy constructor

[tgregg]

https://github.com/amazon-ion/ion-java/blob/master/src/main/java/com/amazon/ion/impl/lite/IonValueLite.java#L305

Currently, this array is allocated even in the following situations:

1. When the array to be copied has length 0
2. When the array to be copied has length > 0 but only null elements

This should be optimized by waiting to allocate the array until at least one non-null element is found, and using a constant SymbolToken[0] when no annotations are present.

Further, we should double-check the logic that causes an array with length > 0 and only null elements to exist, to make sure it is not being wastefully over-allocated.