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)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_94
SMA_SV36
SMA_SV36