fix Converter type indexing for Converters not supported by Extension #2614

Job	Run time
jsonschema	0s
asdf-schemas / Load tox environments	4s
core / Load tox environments	8s
core / Python 3.11 coverage	12m 22s
core / Python 3.10 coverage	13m 18s
core / Python 3.9 coverage	11m 30s
asdf-schemas / asdf-standard (ubuntu-latest)	1m 0s
asdf-schemas / asdf-transform-schemas (ubuntu-latest)	1m 47s
asdf-schemas / asdf-unit-schemas (ubuntu-latest)	42s
test / Load tox environments	8s
compatibility / Load tox environments	8s
oldest / Load tox environments	8s
dev / Load tox environments	9s
package / Build source and wheel distribution	4m 43s
compatibility / compatibility (ubuntu-latest)	3m 5s
test / py39-parallel (macos-latest)	2m 19s
test / py39-parallel (windows-latest)	3m 1s
oldest / py39-oldestdeps-parallel (ubuntu-latest)	1m 48s
dev / py39-devdeps-parallel (ubuntu-latest)	12m 58s
dev / py310-devdeps-parallel (ubuntu-latest)	11m 22s
dev / py311-devdeps-parallel (ubuntu-latest)	12m 15s
	1h 32m 55s

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