Make isclock robust to non TimeDomain argument

Author: pitx-perf

src/clock.jl

@@ -40,7 +40,7 @@ discrete-time systems that assume a fixed sample time, such as PID controllers a
 filters.
 """ SolverStepClock
 
-isclock(c) = @match c begin
+isclock(c) = isa(c, TimeDomain) && @match c begin
     PeriodicClock() => true
     _ => false
 end

test/clock.jl

@@ -19,6 +19,7 @@ using MLStyle: @match
     @test isclock(PeriodicClock(; dt = 1.0))
     @test !isclock(Continuous())
     @test !isclock(SolverStepClock())
+    @test !isclock(nothing)
 
     @test !issolverstepclock(PeriodicClock(; dt = 1.0))
     @test !issolverstepclock(Continuous())