[lldb/DataFormatters] Display null C++ pointers as nullptr

Display null pointer as `nullptr`, `nil` and `NULL` for C++,
Objective-C/Objective-C++ and C respectively. The original motivation
for this patch was to display a null std::string pointer as nullptr
instead of "", but the fix seemed generic enough to be done for all
summary providers.

Differential revision: https://reviews.llvm.org/D77153