[clang] [Clang] prevent null explicit object argument from being deduced (PR #104328)

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cor3ntin wrote:

I am happy with that.
Trying to move the change to `AddMethodTemplateCandidate` would be more complicated than it's worth as we would
still want to add the function to the overload set for diagnostic purposes

https://github.com/llvm/llvm-project/pull/104328