Fix: reinit! error when using AbsTerminationMode

[AJ0070]

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

Fixed an issue where calling reinit! with AbsTerminationMode would throw:

MethodError: no method matching ndims(::Type{Nothing})
undefined

Root Cause

The NonlinearTerminationModeCache for AbsTerminationMode initializes u as nothing, but reinit! attempted to broadcast into it. The fix ensures we only update cache.u when it's not nothing.

Solution

Added guard clause to skip updating cache.u when it's nothing:

if cache.u !== nothing
    #...existing logic...
end

Verification

Added regression test that creates a termination cache with u_unaliased = nothing and verifies reinit! no longer throws:

@testset "reinit! with AbsTerminationMode" begin
    mode = AbsTerminationMode()
    cache = NonlinearTerminationModeCache(
        nothing, ReturnCode.Default, 1e-8, 1e-8, Inf, mode,
        nothing, nothing, 0, nothing, nothing, nothing, nothing, nothing, false
    )
    @test_nowarn SciMLBase.reinit!(cache, [1.0, 1.0], [1.1, 1.1])
end

Resolves #687.