Fix bug when creating an empty Phantom

[pvillacorta]

This PR solves #498

Phantom(x=[]), Phantom{Float64}(x=[]) or Phantom{Float32}(x=[]) are no longer supported*, since the element type of the Phantom is Inferred from its input arguments, and defining x=[] (eltype Any) is not a good practice in Julia.

Now, it is possible to define an empty Phantom this way:

Phantom() # default eltype is Float64
Phantom{Float64}()
Phantom{Float32}()
Phantom{Int64}()
(...)

*The example in this page is updated too

[pvillacorta]

Buildkite CI is still failing because of JuliaGPU/oneAPI.jl#473