Avoid string in case of empty CBOR objects

Add a cmake option to use SYSTEM in target_include_directories

Suppress C4127 warning in unit-json_pointer.cpp

Fix truncation warning

Fix memory leak in to_json

 Conflicts:
	include/nlohmann/detail/output/serializer.hpp
	single_include/nlohmann/json.hpp

Specified git branch for google benchmark fetch in benchmark test

Update documentation

The default branch was renamed from "master" to "main", which breaks the cloning process.

Possibility to use without the dependency to file io and streams to use in intel sgx enclaves

Consistency with `using` in README.md