This will work if you have a focus which is in range, or if you don't have a focus.
/cast [@focus,exists,nodead] Throwdown
If your focus is out of range, you're probably still out of luck, that's one of the decisions a macro can't make for you.
Better yet may be
This will do the throwdown on your focus (if you have one) only if you've got the control key hit too. If you're clicking, change "mod:ctrl" to "btn:2" and right-click your macro to get your focus throwndown.
/cast [@focus,exists,mod:ctrl] Throwdown
Note that this version still has the "focus exists but isn't in range" issue - you can't work around that automatically.