Enable sql function ifnull, nullif and isnull

[harold-wang]

Issue #950 , if available:

Description of changes:
Add flow control functions ifNull, nullIf and isNull (#950)

• Init
• UT
• IT

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #962 (e2f159b) into develop (d419e8e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             develop     #962    +/-   ##
===========================================
  Coverage      99.87%   99.87%            
- Complexity      2347     2396    +49     
===========================================
  Files            232      234     +2     
  Lines           5395     5500   +105     
  Branches         350      357     +7     
===========================================
+ Hits            5388     5493   +105     
  Misses             5        5            
  Partials           2        2            

Impacted Files	Coverage Δ	Complexity Δ
...opendistroforelasticsearch/sql/expression/DSL.java	100.00% <100.00%> (ø)	132.00 <3.00> (+3.00)
...h/sql/expression/function/BuiltinFunctionName.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...ion/operator/predicate/UnaryPredicateOperator.java	100.00% <100.00%> (ø)	23.00 <10.00> (+10.00)
...elasticsearch/sql/analysis/ExpressionAnalyzer.java	100.00% <0.00%> (ø)	33.00% <0.00%> (+2.00%)
...ticsearch/sql/ppl/parser/AstExpressionBuilder.java	100.00% <0.00%> (ø)	28.00% <0.00%> (+2.00%)
...ticsearch/sql/sql/parser/AstExpressionBuilder.java	100.00% <0.00%> (ø)	51.00% <0.00%> (+3.00%)
...ticsearch/sql/planner/physical/WindowOperator.java	100.00% <0.00%> (ø)	11.00% <0.00%> (+1.00%)
...csearch/sql/analysis/SelectExpressionAnalyzer.java	100.00% <0.00%> (ø)	9.00% <0.00%> (ø%)
...csearch/sql/analysis/WindowExpressionAnalyzer.java	100.00% <0.00%> (ø)	13.00% <0.00%> (+1.00%)
...csearch/sql/expression/aggregation/Aggregator.java	100.00% <0.00%> (ø)	9.00% <0.00%> (+6.00%)
... and 16 more

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[chloe-zh]

Could you also update the user manual and add corresponding doctest cases here: https://github.com/opendistro-for-elasticsearch/sql/blob/develop/docs/user/dql/functions.rst#conditional-functions

[dai-chen]

Please add doctest here: https://github.com/opendistro-for-elasticsearch/sql/blob/develop/docs/user/dql/functions.rst#conditional-functions

[dai-chen]

Btw, I missed IF function, could you add it too? You can do it same here or a separate PR. Thanks!

An issue related: #861

[penghuo]

• Please add the documentation accordingly
• Remove the leading "*" in the PR title

[dai-chen]

Just several minor comments. Thanks for your changes!