Fix: don't use nullptr for std::optional<StringRefNull>

`StringRefNull` cannot reference a `nullptr` string, and so
`uiDefAutoButR()` should get a `std::nullopt` instead of `nullptr`.

Pull Request: https://projects.blender.org/blender/blender/pulls/132852

source/blender/editors/animation/anim_channels_defines.cc

@@ -6248,7 +6248,8 @@ void ANIM_channel_draw_widgets(const bContext *C,
                                 &ptr,
                                 prop,
                                 array_index,
-                                RNA_property_type(prop) == PROP_ENUM ? nullptr : "",
+                                RNA_property_type(prop) == PROP_ENUM ? std::nullopt :
+                                                                       std::optional(""),
                                 ICON_NONE,
                                 offset,
                                 rect->ymin,

source/blender/editors/space_outliner/outliner_draw.cc

@@ -1950,7 +1950,7 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
                                     ptr,
                                     prop,
                                     -1,
-                                    (prop_type == PROP_ENUM) ? nullptr : "",
+                                    (prop_type == PROP_ENUM) ? std::nullopt : std::optional(""),
                                     ICON_NONE,
                                     x + pad_x,
                                     te->ys + pad_y,