DoC: specialize union for the case dst=empty and/or delta=empty this avoids O(n^2) insert and becomes O(n)
DoC: specialize union for the case dst=empty and/or delta=empty
this avoids O(n^2) insert and becomes O(n)

Signed-off-by: Nuno Lopes <c61bc7502a9d053e92b2bfd97b87138a818cfe20@microsoft.com>

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