Add FILLNULL command in PPL (#3032)

[normanj-bitquill]

Description

Adds the FILLNULL command for PPL. FILLNULL will replace NULL values in specified fields.

Related Issues

Resolves #3032

Check List

• [Y] New functionality includes testing.
• [Y] New functionality has been documented.
• [Y] New functionality has javadoc added.
• [Y] New functionality has a user manual doc added.
• [N/A] API changes companion pull request created.
• [Y] Commits are signed per the DCO using --signoff.
• [?] Public documentation issue/PR created.

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[normanj-bitquill]

@MaxKsyunz I have added some integration tests.

[normanj-bitquill]

@jduo I have added an explain test.

[normanj-bitquill]

@normanj-bitquill please lets align the docs with the existing ppl fillnull doc in spark

@YANG-DB The docs have been updated to match the docs for Spark. The sample queries are still different, since they are run in this repo.

[normanj-bitquill]

@normanj-bitquill plz fix the keywordsCanBeId list and we can merge

@YANG-DB I have added it.

[acarbonetto]

Can we do this?

• [x] Public documentation issue/PR created.

[YANG-DB]

@normanj-bitquill LGTM

• @acarbonetto @Yury-Fridlyand @MaxKsyunz : can we approve and merge ? thanks