Support NULL literal as function argument

[dai-chen]

Issue #, if available: #927, #715

Description of changes:

1. Add undefined type for NULL literal
2. Change JDBC driver to map undefined type to JDBC NULL type

Documentation: https://github.com/dai-chen/sql/blob/support-null-literal-as-function-argument/docs/user/general/datatypes.rst#undefined-data-type

Design: A new undefined data type is added as narrowest and root of data type tree.

1. UNDEFINED represents only the type of NULL literal for now (probably represent some other special literals in future)
2. UNKNOWN represents type not supported yet. For example, some new Elasticsearch type is given as function argument which yet to be added to core engine.

With this new type, we're able to support NULL literal. For example, given ABS(NULL), the distance between NULL and INTEGER and DOUBLE are 1 and 2 respectively, so function signature ABS(INTEGER) is used to evaluate its value. Previously NULL literal's type is unknown which caused exception thrown.

Testing: Add UT and IT. Because SQLite and H2 has different result type for expression with NULL literal, a NullLiteralIT is added manually.

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 #985 (d7526fd) into develop (47d50d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop     #985   +/-   ##
==========================================
  Coverage      99.87%   99.87%           
- Complexity      2397     2400    +3     
==========================================
  Files            234      234           
  Lines           5513     5517    +4     
  Branches         357      357           
==========================================
+ Hits            5506     5510    +4     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...elasticsearch/sql/data/model/ExprMissingValue.java	100.00% <100.00%> (ø)	9.00 <1.00> (ø)
...forelasticsearch/sql/data/model/ExprNullValue.java	100.00% <100.00%> (ø)	9.00 <1.00> (ø)
...roforelasticsearch/sql/data/type/ExprCoreType.java	100.00% <100.00%> (ø)	13.00 <9.00> (+3.00)
...h/sql/expression/conditional/cases/CaseClause.java	100.00% <100.00%> (ø)	14.00 <2.00> (ø)
...ion/operator/predicate/UnaryPredicateOperator.java	100.00% <100.00%> (ø)	23.00 <0.00> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 47d50d2...d7526fd. Read the comment docs.

[chloe-zh]

LGTM, thanks for the changes!

[harold-wang]

Thanks for the change!