Fix string construction from null pointer.
Fix string construction from null pointer.

saschazelzer/CppMicroServices,CppMicroServices/CppMicroServices,CppMicroServices/CppMicroServices,CppMicroServices/CppMicroServices,ksubramz/CppMicroServices,saschazelzer/CppMicroServices,ksubramz/CppMicroServices,ksubramz/CppMicroServices,ksubramz/CppMicroServices,CppMicroServices/CppMicroServices,ksubramz/CppMicroServices,CppMicroServices/CppMicroServices