Fix ASCII function and groom UT for text functions. (#301)

[Yury-Fridlyand]

Description

Update ASCII function (to return 0 for empty string, see doc)

BEFORE

opensearchsql> select ascii('');
TransportError(503, 'StringIndexOutOfBoundsException', {'error': {'reason': 'There was internal problem at backend', 'details': 'Index 0 out of bounds for length 0', 'type': 'StringIndexOutOfBoundsException'}, 'status': 503})

AFTER

opensearchsql> select ascii('');
fetched rows / total rows = 1/1
+-------------+
| ascii('')   |
|-------------|
| 0           |
+-------------+

Update UT for all text functions (similar to #1310):

• remove null/missing tests
• make tests parametrized

Issues Resolved

N/A

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Merging #1895 (82a333f) into main (bb3c340) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1895   +/-   ##
=========================================
  Coverage     97.52%   97.52%           
- Complexity     4655     4656    +1     
=========================================
  Files           408      408           
  Lines         11952    11953    +1     
  Branches        829      830    +1     
=========================================
+ Hits          11656    11657    +1     
  Misses          289      289           
  Partials          7        7           

Flag	Coverage Δ
sql-engine	97.52% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
...g/opensearch/sql/expression/text/TextFunction.java	100.00% <100.00%> (ø)