Fix updating default value metadata of conditional parameters

[ven-k]

This fixes the bug in populating structure dict for defaults of conditional parameters

Fixes:

julia> a = true; @mtkmodel A begin
                  @parameters begin
                      if a
                          b = 2, [description = "sjodf"]
                      end
                  end
              end
ERROR: LoadError: ArgumentError: invalid index: :b of type Symbol

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

No doc changes are required.

[ven-k]

Model Parsing tests have passed¹

@ChrisRackauckas this is ready to merge.

Footnotes

1. https://github.com/SciML/ModelingToolkit.jl/actions/runs/8108338346/job/22161424946?pr=2438#step:6:1316 ↩