[clang-doc] Add check for pointer validity

clang-doc would SEGV when running over the Fuchsia code base.
This patch adds a check to avoid dereferencing potentially null pointers
in the Values vector. These pointers were either never valid or had been
invalidated when the underlying pointer in std::unique_ptr was moved from,
hence making it nullptr within the vector.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D130279