Rule out OString(std::nullptr_t)

(This is no issue for OUString, as it has two ctors taking a single pointer
argument, so passing a null pointer is ambiguous anyway.)

Change-Id: I36f44b29eb84eb83e284fa080d706eabb4b485d5

include/rtl/string.hxx

@@ -23,6 +23,7 @@
 #include <sal/config.h>
 
 #include <cassert>
+#include <cstddef>
 #include <new>
 #include <ostream>
 #include <string.h>
@@ -256,6 +257,10 @@ public:
     }
 #endif
 
+#ifdef LIBO_INTERNAL_ONLY
+    OString(std::nullptr_t) = delete;
+#endif
+
     /**
       Release the string data.
     */