- we can use EMPTY_DICT only if protocol >= 1

Also: similarly to list (33d1926f), since we are now using EMPTY_DICT
only optionally, it is logical to swit to

	MARK + ... + DICT

from

	EMPTY_DICT (or MARK + DICT @proto=0) + MARK + ... + SETITEMS

which is at least 1 byte longer.

For the reference - SETITEMS is also from protocol 1, while DICT is from
protocol 0.