Fix T93899: Incorrect decorator buttons for geometry nodes vector/colors

For properties exposed to the geometry nodes modifier, decorators didn't
work at all for colors and it only worked on the X component of vectors.
The fix is to use -1 for the RNA index of the decorator button instead
of 1, which lets the UI code figure out what to do with arrays.

source/blender/modifiers/intern/MOD_nodes.cc

@@ -1472,7 +1472,7 @@ static void add_attribute_search_or_value_buttons(const bContext &C,
   }
   else {
     uiItemR(row, md_ptr, rna_path.c_str(), 0, "", ICON_NONE);
-    uiItemDecoratorR(row, md_ptr, rna_path.c_str(), 0);
+    uiItemDecoratorR(row, md_ptr, rna_path.c_str(), -1);
   }
 }