[Ada] Remove redundant checks for empty lists

gcc/ada/

	* exp_ch4.adb (Variant_Is_Unconstrained_UU): Remove redundant
	check for empty list.
	* exp_disp.adb (Find_Entry_Index): Simplify by removing
	redundant check and counting from zero; fix type of a local
	variable.
	* sem_ch12.adb (Save_Global_Descendant): Remove an unnecessary
	special-case for empty lists.