[JENKINS-66826] Fix NPE in ErrorAction.isUnserializableException

[megathaum]

Performing a non null check for MissingPropertyException's type in isUnserializableException to avoid the NPE. See: https://issues.jenkins.io/browse/JENKINS-66826
The NPE is causing the ErrorExtTest#testErrorExtNoGroovyNullPointerException to fail in pipeline-stage-view / pipeline-rest-api.

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