[FLINK-21774][sql-client] Do not display column names when return set is emtpy in SQL Client

[SteNicholas]

What is the purpose of the change

Currently, SQL Client displays the column names when the return set of SHOW MODULES is empty:

SHOW MODULES;
+-------------+
| module name |
+-------------+
0 row in set
!ok

The output of SHOW MODULES could be improved by simply omit the column names header.

Brief change log

• CliClient add callShowResult and callShowCurrentResult method to unify the client execution of SHOW and SHOW statement.

Verifying this change

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• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit c892144 (Mon Mar 15 06:50:16 UTC 2021)

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[wuchong]

Thanks @SteNicholas , I think we can further improvment the column names displaying. Currently, may SHOW command doesn't display column names when there are results. Actualy, many mature database vendors will display column names (print like a table).

I suggest all the SHOW commands should call getResultAsTableauForm instead which already print the column name headers.

[wuchong]

Besides, please rebase to the latest master branch.

[SteNicholas]

@wuchong , thanks for your detailed review. I have followed your suggestion to use getResultAsTableauForm for SHOW command. Please help to review again.

[wuchong]

I only have 2 minor comments.

[SteNicholas]

@wuchong , thanks for the minor comments. I have followed the suggestion for listFunctions and listUserDefinedFunctions. Please check again.

[wuchong]

Thanks for the updating. LGTM.