[llvm] [ADT] Exit doFind on an empty map, not just an unallocated one (PR #220294)

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khaki3 wrote:

The clang speedup decreased after the rebase, likely due to this commit adding more non-empty lookups. It is still an improvement. https://github.com/llvm/llvm-project/commit/1829cfa8da

https://github.com/llvm/llvm-project/pull/220294