[ADT] Fix RadixTree singular iterator use in findOrCreate (#181510)
The root node's Key is initialized with default-constructed (singular)
iterators. When findOrCreate calls llvm::mismatch(Key, Curr->Key) on the
first loop iteration where Curr is the root, these singular iterators
are passed to std::mismatch. _GLIBCXX_DEBUG correctly rejects them as
not forming a valid iterator range.
Skip the mismatch when Curr is the root node since its key is
conceptually empty, making the mismatch a no-op.
Fixes a test failure in ADTTests/RadixTreeTypeTest introduced by
5fda2a5d9c1a ("[NFC][ADT] Add RadixTree (#164524)").
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