Revert "Revert "Prevent nonsensical comparations between OUString and nullptr_t""

This reverts commit e559c0c9; just turn the
deleted overloads into non-friend functions (and rely on any other overloads to
be still found via ADL).

Change-Id: I2af834162cab2e71ed9e32ae6903bc9f86d77ba2
Reviewed-on: https://gerrit.libreoffice.org/30441Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

include/rtl/ustring.hxx

@@ -3226,6 +3226,18 @@ public:
     }
 };
 
+#if defined LIBO_INTERNAL_ONLY
+// Prevent the operator ==/!= overloads with 'sal_Unicode const *' paramter from
+// being selected for nonsensical code like
+//
+//   if (ouIdAttr == nullptr)
+//
+void operator ==(OUString const &, std::nullptr_t) = delete;
+void operator ==(std::nullptr_t, OUString const &) = delete;
+void operator !=(OUString const &, std::nullptr_t) = delete;
+void operator !=(std::nullptr_t, OUString const &) = delete;
+#endif
+
 #if defined LIBO_INTERNAL_ONLY // "RTL_FAST_STRING"
 /// @cond INTERNAL