Avoid NPE on Stream scaling metric

nuxeo            | 2023-04-03T09:08:21,246 ERROR [AbstractComputation] Computation: stream/introspection fails last record: internal-metrics-00:+1011, after retries.
nuxeo            | java.lang.NullPointerException: Cannot invoke "com.fasterxml.jackson.databind.node.ObjectNode.set(String, com.fasterxml.jackson.databind.JsonNode)" because "stream" is null
nuxeo            |      at org.nuxeo.ecm.core.bulk.introspection.StreamIntrospectionConverter.getActiveComputations(StreamIntrospectionConverter.java:442) ~[nuxeo-core-bulk-2023.0-SNAPSHOT.jar:?]
nuxeo            |      at org.nuxeo.ecm.core.bulk.introspection.StreamIntrospectionConverter.getActivity(StreamIntrospectionConverter.java:352) ~[nuxeo-core-bulk-2023.0-SNAPSHOT.jar:?]
nuxeo            |      at org.nuxeo.ecm.core.bulk.introspection.StreamIntrospectionComputation.updateModel(StreamIntrospectionComputation.java:191) ~[nuxeo-core-bulk-2023.0-SNAPSHOT.jar:?]
nuxeo            |      at org.nuxeo.ecm.core.bulk.introspection.StreamIntrospectionComputation.processRecord(StreamIntrospectionComputation.java:162) ~[nuxeo-core-bulk-2023.0-SNAPSHOT.jar:?]
nuxeo            |      at org.nuxeo.lib.stream.computation.log.ComputationRunner.lambda$processRecordWithRetry$10(ComputationRunner.java:510) ~[nuxeo-stream-2023.0-SNAPSHOT.jar:?]

...
nuxeo            | 2023-04-03T09:08:21,253 ERROR [ComputationRunner] stream/introspection: Terminate computation due to previous failure