♻️ Refactor `_CallbackHandler` to handle optional model instances #545

Job	Run time
mypy (3.9)	21s
test py3.8 on ubuntu with extra	45s
mypy (3.10)	18s
test py3.8 on ubuntu with pydantic v2	22s
test py3.9 on ubuntu with extra	42s
mypy (3.11)	17s
test py3.8 on ubuntu with pydantic v1	21s
mypy (3.12)	21s
test py3.8 on macos with pydantic v2	1m 1s
test py3.8 on macos with pydantic v1	1m 9s
test py3.8 on windows with pydantic v2	47s
test py3.8 on windows with pydantic v1	52s
test py3.10 on ubuntu with extra	50s
test py3.9 on ubuntu with pydantic v2	24s
test py3.9 on ubuntu with pydantic v1	23s
test py3.11 on ubuntu with extra	40s
test py3.9 on macos with pydantic v2	28s
test py3.9 on macos with pydantic v1	29s
test py3.12 on ubuntu with extra	51s
test py3.9 on windows with pydantic v2	49s
test py3.10 on ubuntu with pydantic v2	27s
test py3.10 on macos with pydantic v2	28s
test py3.9 on windows with pydantic v1	52s
test py3.10 on windows with pydantic v2	1m 23s
test py3.10 on ubuntu with pydantic v1	25s
test py3.11 on ubuntu with pydantic v2	23s
test py3.10 on macos with pydantic v1	28s
test py3.11 on macos with pydantic v2	28s
test py3.10 on windows with pydantic v1	51s
test py3.11 on windows with pydantic v2	1m 4s
test py3.11 on ubuntu with pydantic v1	30s
test py3.12 on ubuntu with pydantic v2	27s
test py3.11 on macos with pydantic v1	33s
test py3.12 on macos with pydantic v2	32s
test py3.11 on windows with pydantic v1	52s
test py3.12 on windows with pydantic v2	1m 3s
test py3.12 on ubuntu with pydantic v1	32s
test py3.12 on macos with pydantic v1	35s
test py3.12 on windows with pydantic v1	48s
check	8s
	24m 59s

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