fix: fix array variables in mtkmodel

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[baggepinnen]

This PR appears to break something in Multibody.jl, on this branch, the example
https://juliacomputing.github.io/Multibody.jl/dev/examples/wheel/#Slip-based-planar-wheel
fails already upon model instantiation with an error

julia> @named model = TwoTrackWithDifferentialGear()
ERROR: MethodError: no method matching hasmetadata(::Vector{Num}, ::Type{Symbolics.VariableDefaultValue})

Closest candidates are:
  hasmetadata(::Num, ::Any)
   @ Symbolics ~/.julia/packages/Symbolics/XnDVB/src/Symbolics.jl:187
  hasmetadata(::Complex{Num}, ::Any)
   @ Symbolics ~/.julia/packages/Symbolics/XnDVB/src/Symbolics.jl:187
  hasmetadata(::SymbolicUtils.Symbolic, ::Any)
   @ SymbolicUtils ~/.julia/packages/SymbolicUtils/ij6YM/src/types.jl:631

Stacktrace:
  [1] hasdefault(v::Vector{Num})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/utils.jl:237
  [2] collect_defaults!(defs::Dict{Any, Any}, vars::Vector{Any})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/utils.jl:255
  [3] process_variables!(var_to_name::Dict{Any, Any}, defs::Dict{Any, Any}, vars::Vector{Any})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/utils.jl:248
  [4] ODESystem(deqs::Vector{…}, iv::Num, dvs::Vector{…}, ps::Vector{…}; controls::Vector{…}, observed::Vector{…}, systems::Vector{…}, tspan::Nothing, name::Symbol, default_u0::Dict{…}, default_p::Dict{…}, defaults::Dict{…}, guesses::Dict{…}, initializesystem::Nothing, initialization_eqs::Vector{…}, schedule::Nothing, connector_type::Nothing, preface::Nothing, continuous_events::Nothing, discrete_events::Nothing, parameter_dependencies::Vector{…}, checks::Bool, metadata::Nothing, gui_metadata::ModelingToolkit.GUIMetadata)
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/diffeqs/odesystem.jl:245
  [5] #__FixedTranslation__#11
    @ ~/.julia/packages/UnPack/EkESO/src/UnPack.jl:705 [inlined]
  [6] __FixedTranslation__(; name::Symbol, r::Vector{Int64}, radius::Nothing, render::Nothing, phi::Nothing, w::Nothing)
    @ Multibody.PlanarMechanics ./none:0
  [7] __FixedTranslation__
    @ ./none:0 [inlined]
  [8] #_#411
    @ ~/.julia/dev/ModelingToolkit/src/systems/model_parsing.jl:25 [inlined]
  [9] macro expansion
    @ ~/.julia/dev/ModelingToolkit/src/systems/abstractsystem.jl:1950 [inlined]
 [10] 
    @ Main ~/.julia/dev/ModelingToolkit/src/systems/model_parsing.jl:138
 [11] __TwoTrackWithDifferentialGear__
    @ ~/.julia/dev/ModelingToolkit/src/systems/model_parsing.jl:138 [inlined]
 [12] #_#411
    @ ~/.julia/dev/ModelingToolkit/src/systems/model_parsing.jl:25 [inlined]
 [13] top-level scope
    @ ~/.julia/dev/ModelingToolkit/src/systems/abstractsystem.jl:1950
Some type information was truncated. Use `show(err)` to see complete types.

the example works on MTK master