[clang-tools-extra] [clangd] Fix clangd call hierarchy crash (PR #179718)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 approved this pull request.

Looks good, thanks for the fix!

For good measure, I looked through the other uses of `ObjectMapper` in Protocol.cpp to see if any others were missing this check, but the other uses all have it.

https://github.com/llvm/llvm-project/pull/179718