Allow constructing ideals with no generators

[joschmitt]

There is the following inconvenience when working with ideals in multivariate polynomial rings (MPolyIdeal):

julia> R, (x, y) = PolynomialRing(QQ, [ "x", "y" ])
(Multivariate Polynomial Ring in x, y over Rational Field, fmpq_mpoly[x, y])

julia> ideal(R, elem_type(R)[])
ERROR: AssertionError: length(g) > 0
Stacktrace:
 [1] ideal(g::Vector{fmpq_mpoly})
   @ Oscar ~/.julia/dev/Oscar/src/Rings/mpoly-ideals.jl:40
 [2] ideal(R::FmpqMPolyRing, g::Vector{fmpq_mpoly})
   @ Oscar ~/.julia/dev/Oscar/src/Rings/mpoly-ideals.jl:46
 [3] top-level scope
   @ REPL[33]:1

I would expect this to give the zero ideal in R instead (the ideal with no generators).
Note that I of course do not expect ideal(elem_type(R)[]) to do anything but error.

[joschmitt]

This is settled by #821 as far as I'm concerned. Feel free to close the issue.