ada: Explicitly analyze and expand null array aggregates

ada: Explicitly analyze and expand null array aggregates

Null array aggregates have present but empty lists of expressions and
component associations. This confuses the previous code for ordinary
array aggregates, which assumes that if a list of either expressions or
component associations is present, then it is non-empty.

This patch adds explicit handling for null array aggregates to avoid
assertion failures in code for ordinary array aggregates.

gcc/ada/

	* exp_aggr.adb (Build_Array_Aggr_Code): Don't build aggregate code
	for null array aggregates.
	* sem_aggr.adb (Resolve_Array_Aggregate): Don't examine formatting
	of a null array aggregate.