ESQL: partial failure in few KNN tests

[bpintea]

Description

The following tests fail when introducing assertions that no spec-based test succeeds with (unexpected) partial results:

• knn-function.TestKnnWithNonPushableDisjunctionsOnComplexExpressions
• knn-function.TestKnnWithNonPushableDisjunctions
• knn-function.KnnSearchWithSimilarityOption

In all cases, the partial failure reason is failed to create query: Cannot invoke "java.lang.Integer.intValue()" because "this.dims" is null.

#129293 will mute these tests, this issue is to investigate if these failures are expected or not.

Repro lines (specific to #129293, but the failures aren't):

• ./gradlew ":x-pack:plugin:esql:qa:server:multi-clusters:v9.1.0#newToOld" -Dtests.class="org.elasticsearch.xpack.esql.ccq.MultiClusterSpecIT" -Dtests.method="test {knn-function.TestKnnWithNonPushableDisjunctionsOnComplexExpressions}" -Dtests.seed=A5D8467778903339 -Dtests.bwc=true -Dtests.locale=kam -Dtests.timezone=America/Aruba -Druntime.java=24
• ./gradlew ":x-pack:plugin:esql:qa:server:multi-clusters:v9.1.0#newToOld" -Dtests.class="org.elasticsearch.xpack.esql.ccq.MultiClusterSpecIT" -Dtests.method="test {knn-function.TestKnnWithNonPushableDisjunctions}" -Dtests.seed=A5D8467778903339 -Dtests.bwc=true -Dtests.locale=kam -Dtests.timezone=America/Aruba -Druntime.java=24
• ./gradlew ":x-pack:plugin:esql:qa:server:multi-clusters:v9.1.0#newToOld" -Dtests.class="org.elasticsearch.xpack.esql.ccq.MultiClusterSpecIT" -Dtests.method="test {knn-function.KnnSearchWithSimilarityOption}" -Dtests.seed=A5D8467778903339 -Dtests.bwc=true -Dtests.locale=kam -Dtests.timezone=America/Aruba -Druntime.java=24

One _cluster content example:
java.lang.AssertionError: unexpected partial results: _clusters={details={remote_cluster={_shards={total=1, failed=1, successful=0, skipped=0}, took=34, indices=colors, failures=[{node=null, reason={reason=failed to create query: Cannot invoke "java.lang.Integer.intValue()" because "this.dims" is null, index_uuid=OBZX77CbS6KU2F5YeAJ21w, caused_by={reason=Cannot invoke "java.lang.Integer.intValue()" because "this.dims" is null, type=null_pointer_exception}, index=remote_cluster:colors, type=query_shard_exception}, index=remote_cluster:colors, shard=0}], status=partial}}}

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)