`@system` not compatible with Julia nightly (v1.13)

[schillic]

When no multiplication (*) is found, the @system macro breaks in Julia nightly.

julia> @system(x' = A*x)
# works

julia> @system(x' = Ax)
ERROR: ArgumentError: the entry () does not match a `MathematicalSystems.jl` structure

julia> @system(x' = 0, dim:2)
ERROR: ArgumentError: the entry () does not match a `MathematicalSystems.jl` structure