Make universal polynomial ring play nice with all flint rings

[thofma]

See https://github.com/Nemocas/AbstractAlgebra.jl/pull/1286.

We should make the universal polynomial ring also work flint rings by always creating it with one dummy variable, which should be ignored/handled by subsequent computations.

[fingolfin]

We use the universal polynomial rings over Nemo rings quite a bit in GenericCharacterTables.jl (and a few issues were found and fixed due to this by @SoongNoonien and @lgoettgens, I think).

Unless there is a specific issue you are aware of, I'd suggest we close this as resolved. (Of course there can still be bugs but that's true for all our code)