If the Defending Pokemon doesn't have any Energy cards attached to it, does Typhlosion's "Flare Destroy" attack still require Typhlosion to discard an Energy from itself?
Yes, if the attacking Pokemon has any Energy cards attached, it must discard one of them.
Source: HeartGold & SoulSilver FAQ (2010-02-11), PUI Rules Team (2010-02-11)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS1_110
XY8_20