[clang] Fix an out-of-bound crash when checking template partial specializations. (#86794)

I found this issue (a separate one) during the investigation of #86757,
the crash is similar in substituteParameterMappings, but at different
inner places.

This was an out-of-bound issue where we access front element in an empty
written template argument list to get the instantiation source range.
This patch fixes it by adding a proper guard.