fix(float): ensure relative window grid is allocated (#34459)

Problem:  Uninitialized grid for a "win" relative float when redrawing
          just after it has been opened.
Solution: Ensure window grid is allocated or assigned the default grid.

src/nvim/window.c

@@ -844,6 +844,7 @@ void ui_ext_win_position(win_T *wp, bool validate)
         }
         int row_off = 0;
         int col_off = 0;
+        win_grid_alloc(win);
         grid = grid_adjust(&win->w_grid, &row_off, &col_off);
         row += row_off;
         col += col_off;