Allow get to return non-nothing

[blegat]

I looked at how to fix oxfordcontrol/COSMO.jl#185 and it starts being too hacky.
The time limit can be tweaked with two attributes, RawOptimizerAttribute("time_limit") or MOI.TimeLimitSec. Both attribute actually modify the same thing so they are kept in sync. Asking for get to return nothing is maybe a bit too much:

MOI.set(model, RawOptimizerAttribute("time_limit"), 0.0)
MOI.get(model MOI.TimeLimitSec()) # what should this return ?
MOI.set(model, MOI.TimeLimitSec(), nothing)
MOI.get(model MOI.TimeLimitSec()) # returns `nothing`
MOI.set(model, RawOptimizerAttribute("time_limit"), 0.0)
MOI.get(model MOI.TimeLimitSec()) # what should this return ?
MOI.set(model, RawOptimizerAttribute("time_limit"), 1.0)
MOI.get(model MOI.TimeLimitSec()) # what should this return ?

[odow]

I'm okay with this. I've already added it to #2180.