[clang] Place strdup in the unix.cstring.NullArg list (PR #106387)

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fawdlstty wrote:

@steakhal Sorry, I didn't know there was a static checker. I think it should be noted in the documentation for functions whose arguments cannot be 0. May I should check these two functions in the static checker or if I should turn off the pr?

https://github.com/llvm/llvm-project/pull/106387