disable pop up menu when pass in empty array

[haolun1996]

able to return empty array on buildMenuItems on class which implements PlutoColumnMenuDelegate by putting on field columnMenuDelegate and checking the items if it is empty then wont returning for showMenu

In short, putting empty array then pop up menu won't be shown

[bosskmk]

If the column right menu is not displayed, you can do as follows.

PlutoColumn(enableContextMenu: false, );

Because of this, it seems that the intention of this PR is redundant. Please let me know if you have other use cases that need to be covered by a PR. thank you