Fix Tree focus border disappears when Border Size is set to 0

(cherry picked from commit 4c0ab0780936538bf9fb4bf2f11f61eafdce76d2)

editor/editor_themes.cpp

@@ -827,7 +827,7 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 
 	Ref<StyleBoxFlat> style_tree_cursor = style_default->duplicate();
 	style_tree_cursor->set_draw_center(false);
-	style_tree_cursor->set_border_width_all(border_width);
+	style_tree_cursor->set_border_width_all(MAX(1, border_width));
 	style_tree_cursor->set_border_color(contrast_color_1);
 
 	Ref<StyleBoxFlat> style_tree_title = style_default->duplicate();