Simplify ON keyword check to only needed case

[predictor2718]

Summary

• SET @@variable = ON failed with "Missing expression" because ON is a reserved keyword that
  Expression::parse() rejected
• OFF worked because it is not a reserved keyword
• Fix: in SetOperation::parse(), after Expression::parse() returns null, accept ON as a literal value since
  it is valid in SET statements

Fixes #627

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.90%. Comparing base (4c45eac) to head (0bd40a6).

Files with missing lines	Patch %	Lines
src/Components/SetOperation.php	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             5.11.x     #652      +/-   ##
============================================
- Coverage     96.92%   96.90%   -0.02%     
- Complexity     2316     2319       +3     
============================================
  Files            71       71              
  Lines          5110     5112       +2     
============================================
+ Hits           4953     4954       +1     
- Misses          157      158       +1     

Flag	Coverage Δ
unit-7.2-ubuntu-latest	96.90% <66.66%> (-0.02%)	⬇️
unit-7.3-ubuntu-latest	96.89% <66.66%> (-0.02%)	⬇️
unit-7.4-ubuntu-latest	96.87% <66.66%> (-0.02%)	⬇️
unit-8.0-ubuntu-latest	96.89% <66.66%> (-0.02%)	⬇️
unit-8.1-ubuntu-latest	96.89% <66.66%> (-0.02%)	⬇️
unit-8.2-ubuntu-latest	96.85% <66.66%> (-0.02%)	⬇️
unit-8.3-ubuntu-latest	96.85% <66.66%> (-0.02%)	⬇️
unit-8.4-ubuntu-latest	96.85% <66.66%> (-0.02%)	⬇️
unit-8.5-ubuntu-latest	96.85% <66.66%> (-0.02%)	⬇️

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