[Prism]: Fix unknown coder urn key: beam:coder:nullable:v1

[damondouglas]

This PR fixes #31336. I performed the following to validate this change:

1. Run Prism server.

./gradlew :runners:prism:runServer

2. Execute :runners:portability:java:ulrLoopbackValidatesRunnerTests on Prism:

TEST=org.apache.beam.sdk.transforms.CombineFnsTest
./gradlew :runners:portability:java:ulrLoopbackValidatesRunnerTests -PjobEndpoint=localhost:8073 --tests="$TEST"

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[lostluck]

LGTM