ES|QL: Fix VectorSimilarityFunctionsIT

[ioanatia]

fixes #133059

Error details:

The tests fail with this type of error:

VectorSimilarityFunctionsIT > testSimilarityBetweenVectors {functionName=v_l2_norm similarityFunction=org.elasticsearch.xpack.esql.vector.VectorSimilarityFunctionsIT$$Lambda/0x00000e00003db5d0@48df4071} FAILED
    java.lang.IllegalStateException: Block [FloatVectorBlock[vector=ConstantFloatVector[positions=0, value=0.0]]] does not have same position count: 0 != 1
        at org.elasticsearch.compute.data.Page.<init>(Page.java:88)
        at org.elasticsearch.compute.data.Page.appendBlock(Page.java:162)
        at org.elasticsearch.compute.operator.EvalOperator.process(EvalOperator.java:46)
        at org.elasticsearch.compute.operator.AbstractPageMappingOperator.getOutput(AbstractPageMappingOperator.java:85)
        at org.elasticsearch.compute.operator.Driver.runSingleLoopIteration(Driver.java:272)
        at org.elasticsearch.compute.operator.Driver.run(Driver.java:186)
        at org.elasticsearch.compute.operator.Driver$1.doRun(Driver.java:420)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at org.elasticsearch.compute.operator.DriverScheduler$1.doRun(DriverScheduler.java:57)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at org.elasticsearch.common.util.concurrent.TimedRunnable.doRun(TimedRunnable.java:35)
        at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:1067)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1095)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:619)
        at java.base/java.lang.Thread.run(Thread.java:1447)

when this happens, we are not releasing the blocks so the next tests fail with :

    java.lang.AssertionError: Request breaker not reset to 0 on node: node_s0
    Expected: <0L>
         but: was <64L>
        at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)

Fix

In the VectorSimilarityFunction evaluator, we first try and detect the vector dimensions by checking the left vector.
We need the vector dimension to instantiate the leftScratch and rightScratch float arrays which we reuse for each row.
In the case where the leftVector has only null values (0 dimensions) - we can return early, since in this case the similarity function should just return null values (as per #132919).
This is where the bug resides, we return a constant float block with a single position (and 0 value, not null).
Changing this to return a constant null block with the right positionCount should fix the issue.

[elasticsearchmachine]

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[elasticsearchmachine]

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