you absolutely do have to have a target in order to use the ability.

>602.2  To activate an ability is to put it onto the stack and pay its costs,
> so that it will even tually resolve and have its effect. Only an
> object’s controller (or its owner, if it doesn’t have a controller)
> can activate its activated ability unless the object specifically says
> otherwise. Activating an ability follows the steps listed below, in
> order. **If, at any point during the activation of an ability, a player
> is unable to comply with any of those steps, the activation is
> illegal; the game returns to the moment before that ability started to
> be activated (see rule 717, “Handling Illegal Actions”)**. Announcements
> and payments can’t be altered after they’ve been made.

----
And you have to choose a target in order to cast a spell or activate an ability

> 601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires. A spell
> may require some targets only if an alternative or additional cost
> (such as a buyback or kicker cost), or a particular mode, was chosen
> for it; otherwise, the spell is cast as though it did not require
> those targets. If the spell has a variable number of targets, the
> player announces how many targets he or she will choose before he or
> she announces those targets. The same target can’t be chosen multiple
> times for any one instance of the word “target” on the spell. However,
> if the spell uses the word “target” in multiple places, the same
> object, player, or zone can be chosen once for each instance of the
> word “target” (as long as it fits the targeting criteria). If any
> effects say that an object or player must be chosen as a target, the
> player chooses targets so that he or she obeys the maximum possible
> number of such effects without violating any rules or effects that say
> that an object or player can’t be chosen as a target. The chosen
> players, objects, and/or zones each become a target of that spell.
> (Any abilities that trigger when those players, objects, and/or zones
> become the target of a spell trigger at this point; they’ll wait to be
> put on the stack until the spell has finished being cast.)

--- 

> 602.2b The remainder of the process for activating an ability is identical to the process for casting a spell listed in rules 601.2b –
> h. Those rules apply to activating an a bility just as they apply to
> casting a spell. An activated ability’s analog to a spell’s mana cost
> (as referenced in rule 601.2e) is its activation cost.


  [1]: http://www.wizards.com/magic/tcg/article.aspx?x=magic/rules