Cleanup: Remove unnecessary std::string braces for default construction

std::string has a default constructor, so this is unnecessary.

source/blender/editors/include/UI_grid_view.hh

@@ -197,7 +197,7 @@ class PreviewGridItem : public AbstractGridViewItem {
   bool hide_label_ = false;
 
  public:
-  std::string label{};
+  std::string label;
   int preview_icon_id = ICON_NONE;
 
   PreviewGridItem(StringRef identifier, StringRef label, int preview_icon_id);

source/blender/editors/include/UI_tree_view.hh

@@ -180,7 +180,7 @@ class AbstractTreeViewItem : public AbstractViewItem, public TreeViewItemContain
 
  protected:
   /** This label is used as the default way to identifying an item within its parent. */
-  std::string label_{};
+  std::string label_;
 
  public:
   /* virtual */ ~AbstractTreeViewItem() override = default;