cmake-gui: Fix handling of missing CMAKE_EXTRA_GENERATOR (#14804)

Since commit 1a1b737c (stringapi: Use strings for generator names,
2014-02-24) cmExternalMakefileProjectGenerator::CreateFullGeneratorName
expects a std::string instead of const char*.  When no extra generator
name is available, pass an empty string to avoid NULL dereference.
Merge topic 'string-apis-fix-cmake-gui' into next

ae32622b cmake-gui: Fix handling of missing CMAKE_EXTRA_GENERATOR (#14804)