Assertion fails when building event callbacks

[DaniGlez]

When composing a system with MTK (8.7.0), empty callbacks (i.e. of the form ModelingToolkit.SymbolicContinuousCallback(Equation[], Equation[])) are sometimes created. These callbacks seem to trip the assertion on

ModelingToolkit.jl/src/systems/diffeqs/abstractodesystem.jl

Line 221 in 7945b36

@assert maximum(eq_ind2affect) == length(affect_functions)

Here's code for reproducing the issue: https://gist.github.com/DaniGlez/22c98bd7d8b3bba479cf98082c62da56
In this example, four callbacks end up in the system after structural_simplify():

4-element Vector{ModelingToolkit.SymbolicContinuousCallback}: ModelingToolkit.SymbolicContinuousCallback(Equation[], Equation[]) ModelingToolkit.SymbolicContinuousCallback(Equation[m₊x(t) ~ 1.0], Equation[m₊v(t) ~ -m₊v(t)]) ModelingToolkit.SymbolicContinuousCallback(Equation[m₊x(t) ~ -1.0], Equation[m₊v(t) ~ -m₊v(t)]) ModelingToolkit.SymbolicContinuousCallback(Equation[], Equation[])

The assertion mentioned above therefore fails, since the largest index mapping to an affect is 3 while there are four affect functions (even if the last one is a dummy function)

[DaniGlez]

BTW, I'm not sure if this is necessarily the preferred solution going forward, but it seems that the issue can be bypassed (at least in this example) with a simple system transformation before the call to solve(...):

using ModelingToolkit: @set!, get_continuous_events, AbstractODESystem
function remove_empty_callbacks(sys::AbstractODESystem)
    cbs = get_continuous_events(sys)
    @set! sys.continuous_events = filter(c -> length(c.eqs) > 0, cbs)
    return sys
end

ssys = remove_empty_callbacks(ssys)