UnionArrayBuilder.appendValue

Append an element to the union array.

If builder is #GArrowDenseUnionArrayBuilder, this must be followed by an append to the appropriate child builder.

If builder is #GArrowSparseUnionArrayBuilder, this must be followed by appends to all child builders. The corresponding child builder must be appended to independently after this method is called, and all other child builders must have null or empty value appended.