Method isNull

Sming/Wiring/WString.h:838–841

String is Null (invalid) by default, i.e. non-SSO and null buffer

	// String is Null (invalid) by default, i.e. non-SSO and null buffer
	__forceinline bool isNull() const
	{
		return !sso.set && (ptr.buffer == nullptr);
	}

	// Get writeable buffer pointer
	__forceinline char* buffer()