Attacks » Sparkling Wish
Can I use Diancie's "Sparkling Wish" Attack to search your deck for an Evolution card even if you have no valid targets to evolve?
Yes, you can, but only because it is an attack. You do as much of the attack as you can, so you can search through your deck but nothing more.
Source: TPCi Rules Team (2017-08-31)
Attacks » Sparkling Wish
Can I use Diancie's "Sparkling Wish" attack to evolve a Pokemon that I just put into play this turn?
Yes, you can do that.
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)