Fix nested list in buildInputs

[Squamto]

Fixes #90

[Lxtharia]

If anyone can give me an approval that this works, I can merge this.

@Aozora-Wings feel free to test this :)

[Artturin]

in mkDerivation conditional values shouldn't (usually) go in // because that doesn't perform deep merging and is a footgun for the future when others add args before // and they get overriden.

When you see a mkDerivation ({} // optionalAttrs x {}) in nixpkgs it's usually because something had to be fixed but doing it properly would have caused too many rebuilds

[Squamto]

Okay thanks for the insight.
I initially debated using optionals and optionalString, but thought optionalAttrs looks like a cleaner solution, especially because I did not want to override any default buildPhase with nothing in the case there is no custom splash.
I will implement the proposed solution, when I am back home later today.