Fix StandardDeviationPostAggregator to account for null return values from the NoopAggregator

[LakshSingla]

Description

Fixes NPE which can arise if the StandardDeviationPostAggregator is passed in null. This adds a check to make sure that the object is the desired type before casting.

The following query fails before the patch, however, please post it.

{
  "queryType": "groupBy",
  "dataSource": {
    "type": "table",
    "name": "wikipedia"
  },
  "intervals": {
    "type": "intervals",
    "intervals": [
      "1998-01-09T23:55:00.000Z/1999-01-10T00:55:00.000Z"
    ]
  },
  "granularity": {
    "type": "all"
  },
  "dimensions": [],
  "aggregations": [
    {
      "type": "variance",
      "name": "preAggregationValue",
      "fieldName": "commentLength",
      "estimator": null,
      "inputType": "float"
    }
  ],
  "postAggregations": [
    {
      "type": "stddev",
      "name": "value",
      "fieldName": "preAggregationValue",
      "estimator": null
    }
  ],
  "limitSpec": {
    "type": "NoopLimitSpec"
  },
  "context": {
    "enableTimeBoundaryPlanning": "true",
    "useCache": true,
    "debug":true
  }
}

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Key changed/added classes in this PR

• MyFoo
• OurBar
• TheirBaz

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.