Add test for iterating over MDNode operands when they are empty (#80737)

With e8512786 the for loop that iterates
over MDNode operands was changed to a range-based for loop. This change
surfaces a bug where if the result of MD->operands() is an ArrayRef that
has a size of 0, then iterating over that ArrayRef leads to a
segmentation fault, due to accessing invalid addresses.
    
This was reverted with 6ce03ff3 but this
test should be added to test that codepath in the future.