fix change of var for sys with no equations

[aml5600]

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

In the case were a system had no equations, like with RealInput/Output, performing map(transform, get_eqs(sys)) would return Any[], because it seemed like the type from transform(ex) couldn't be deduced. This small addition ensures that typeof(ex) === typeof(transform(ex)) and solves this issue. A small test is added which would fail prior to the change.

[aml5600]

Also, it seemed like with JuliaFormatter v2.1.2 fails, and changes many files, with: using JuliaFormatter, ModelingToolkit; format(joinpath(dirname(pathof(ModelingToolkit)), ".."))