attempt to fix NULL ambiguity

remote debugging for build server error:
/var/lib/jenkins/jobs/elektra-multiconfig-gcc-stable/workspace/build_type/RelWithDebInfo/gcc_version/4.4/plugins/ALL/source/src/bindings/cpp/examples/cpp_example_hierarchy.cpp:180:
error: call of overloaded ‘Key(int)’ is ambiguous
/var/lib/jenkins/jobs/elektra-multiconfig-gcc-stable/workspace/build_type/RelWithDebInfo/gcc_version/4.4/plugins/ALL/source/src/bindings/cpp/include/key.hpp:287:
note: candidates are: kdb::Key::Key(std::string, ...)
/var/lib/jenkins/jobs/elektra-multiconfig-gcc-stable/workspace/build_type/RelWithDebInfo/gcc_version/4.4/plugins/ALL/source/src/bindings/cpp/include/key.hpp:262:
note:                 kdb::Key::Key(const char*, ...)
/var/lib/jenkins/jobs/elektra-multiconfig-gcc-stable/workspace/build_type/RelWithDebInfo/gcc_version/4.4/plugins/ALL/source/src/bindings/cpp/include/key.hpp:248:
note:                 kdb::Key::Key(const kdb::Key&)
/var/lib/jenkins/jobs/elektra-multiconfig-gcc-stable/workspace/build_type/RelWithDebInfo/gcc_version/4.4/plugins/ALL/source/src/bindings/cpp/include/key.hpp:220:
note:                 kdb::Key::Key(ckdb::Key*)