[Bridges] fix print_active_bridges for some constrained variable cases

[odow]

Tried to debug jump-dev/ParametricOptInterface.jl#201
Ran into jump-dev/Ipopt.jl#528

Found these.

With this

julia> import MathOptInterface as MOI

julia> import ParametricOptInterface as POI

julia> import SCS

julia> begin
           inner = POI.Optimizer(SCS.Optimizer)
           model = MOI.Bridges.full_bridge_optimizer(inner, Float64)
           p, _ = MOI.add_constrained_variable(model, MOI.Parameter(1.0))
           MOI.Bridges.print_active_bridges(model)
       end
 * Supported objective: MOI.ScalarAffineFunction{Float64}
 * Supported variable: MOI.Parameter{Float64}

where previously

julia> import MathOptInterface as MOI

julia> import ParametricOptInterface as POI

julia> import SCS

julia> begin
           inner = POI.Optimizer(SCS.Optimizer)
           model = MOI.Bridges.full_bridge_optimizer(inner, Float64)
           p, _ = MOI.add_constrained_variable(model, MOI.Parameter(1.0))
           MOI.Bridges.print_active_bridges(model)
       end
 * Supported objective: MOI.ScalarAffineFunction{Float64}
ERROR: KeyError: key (MathOptInterface.Parameter{Float64},) not found
Stacktrace:
 [1] getindex
   @ ~/.julia/packages/OrderedCollections/Xihhq/src/ordered_dict.jl:387 [inlined]
 [2] print_active_bridges(io::Base.TTY, b::MathOptInterface.Bridges.LazyBridgeOptimizer{…}, ::Type{…}, ::Type{…}, offset::String)
   @ MathOptInterface.Bridges ~/git/jump-dev/MathOptInterface/src/Bridges/debug.jl:664
 [3] print_active_bridges(io::Base.TTY, b::MathOptInterface.Bridges.LazyBridgeOptimizer{…}, ::Type{…}, ::Type{…})
   @ MathOptInterface.Bridges ~/git/jump-dev/MathOptInterface/src/Bridges/debug.jl:641
 [4] print_active_bridges(io::Base.TTY, b::MathOptInterface.Bridges.LazyBridgeOptimizer{ParametricOptInterface.Optimizer{…}})
   @ MathOptInterface.Bridges ~/git/jump-dev/MathOptInterface/src/Bridges/debug.jl:550
 [5] print_active_bridges(::MathOptInterface.Bridges.LazyBridgeOptimizer{ParametricOptInterface.Optimizer{…}})
   @ MathOptInterface.Bridges ~/git/jump-dev/MathOptInterface/src/Bridges/debug.jl:556
 [6] top-level scope
   @ REPL[23]:5
Some type information was truncated. Use `show(err)` to see complete types.