fix ambiguity for Random.seed!(rng, nothing)

[marius311]

Prior to this PR we had that:

julia> Random.seed!(Random.default_rng(), nothing)
MersenneTwister(...)

julia> Random.seed!(CuArrays.CURAND.generator(), nothing)
ERROR: MethodError: seed!(::CuArrays.CURAND.RNG, ::Nothing) is ambiguous. Candidates:
  seed!(rng::CuArrays.CURAND.RNG, seed) in CuArrays.CURAND at /home/marius/.julia/packages/CuArrays/l0gXB/src/rand/random.jl:44
  seed!(rng::AbstractRNG, ::Nothing) in Random at /home/marius/src/julia-1.4/usr/share/julia/stdlib/v1.4/Random/src/Random.jl:426

Now the second one works as expected.

I wasn't sure what to make of the calls to Random.seed! outside of a @test but I put the extra one with them, let me know if that's ok.

[maleadt]

Thanks! I guess this would also need to be added to the GPUArrays RNG, for completeness?
bors try

[bors]

try

Build failed:

• ci/gitlab/trying