Improve error msg when trying to solve an empty model

[guimarqu]

Describe the bug

Coluna
Version 0.3.3 - https://github.com/atoptima/Coluna.jl
┌ Warning: No initial primal bound and no cost for global artificial variables.
│ Cost of global artificial variables set to 100000.0
└ @ Coluna ~/.julia/packages/Coluna/JfyZY/src/optimize.jl:20
┌ Warning: No initial primal bound and no cost for local artificial variables.
│ Cost of local artificial variables set to 10000.0
└ @ Coluna ~/.julia/packages/Coluna/JfyZY/src/optimize.jl:33
ERROR: LoadError: MethodError: no method matching getroot(::Nothing)
Closest candidates are:
  getroot(::BlockDecomposition.Tree) at /Users/guimarqu/.julia/packages/BlockDecomposition/OYIcC/src/tree.jl:18
Stacktrace:
 [1] reformulate!(::Coluna.MathProg.Problem, ::Coluna.Annotations) at /Users/guimarqu/.julia/packages/Coluna/JfyZY/src/decomposition.jl:460
 [2] optimize!(::Coluna.MathProg.Problem, ::Coluna.Annotations, ::Coluna.Params) at /Users/guimarqu/.julia/packages/Coluna/JfyZY/src/optimize.jl:54
 [3] optimize!(::Coluna.Optimizer) at /Users/guimarqu/.julia/packages/Coluna/JfyZY/src/MOIwrapper.jl:82
 [4] optimize!(::Model, ::Nothing; bridge_constraints::Bool, ignore_optimize_hook::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/guimarqu/.julia/packages/JuMP/qhoVb/src/optimizer_interface.jl:130
 [5] optimize!(::Model) at /Users/guimarqu/.julia/packages/BlockDecomposition/OYIcC/src/BlockDecomposition.jl:51
 [6] optimize!(::Model, ::Nothing; bridge_constraints::Bool, ignore_optimize_hook::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/guimarqu/.julia/packages/JuMP/qhoVb/src/optimizer_interface.jl:120
 [7] optimize! at /Users/guimarqu/.julia/packages/JuMP/qhoVb/src/optimizer_interface.jl:106 [inlined] (repeats 2 times)

To Reproduce
Try to optimize! an empty BlockModel

Expected behavior
A warning saying that the model is empty