fix: handle observeds in `get_all_timeseries_indexes` for `split = false` systems#3221

ChrisRackauckas merged 2 commits intoSciML:masterSciML/ModelingToolkit.jl:masterfrom

AayushSabharwal:as/nosplit-timeseriesAayushSabharwal/ModelingToolkit.jl:as/nosplit-timeseriesCopy head branch name to clipboard