Fix ASCII function and groom UT for text functions.

[Yury-Fridlyand]

Description

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

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})

Update UT for all text functions (similar to #198):

• 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

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@@                Coverage Diff                 @@
##             integ-fix-ascii     #301   +/-   ##
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  Coverage                   ?   97.39%           
  Complexity                 ?     4609           
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  Files                      ?      401           
  Lines                      ?    11409           
  Branches                   ?      843           
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  Hits                       ?    11112           
  Misses                     ?      290           
  Partials                   ?        7           

Flag	Coverage Δ
sql-engine	97.39% <0.00%> (?)

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