`MaybeRemoveEmptyCrcNode()`. Otherwise appending a `CordBuffer` an empty Cord
with a CRC node crashes (`RemoveCrcNode()` which increases the refcount of a
nullptr child).

Cosmetics: in `Cord::InlineRep::AppendArray()`, return early for empty `src`
before removing the empty CRC node.
PiperOrigin-RevId: 619107278
Change-Id: I4f1bc6b75c662f4678c61e3ef310e8597d62e2e1