Fix alter column with default null

[Tigrov]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	yiisoft/db-migration#11

[what-the-diff]

PR Summary

• Simplified Query Building Operation
  The code in file src/DDLQueryBuilder.php was updated. Previously, a specific line was forcing the system to drop certain constraints for a database column, this operation has now been removed. This essentially simplifies the process for building database queries because we're not unnecessarily removing parts of the database structure anymore.

• Enhancements in Database Column Tests
  In tests/CommandTest.php, important updates were made to provide better testing scenarios. A new test method testAlterColumnWithDefaultNull was introduced. This new test checks if the system behaves correctly when a column in the database that is supposed to have a default value of null is altered. This update ensures our software remains robust when changes are made to this kind of database columns.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9f3d91c) 98.48% compared to head (cb1d144) 98.48%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #282   +/-   ##
=========================================
  Coverage     98.48%   98.48%           
  Complexity      205      205           
=========================================
  Files            15       15           
  Lines           596      596           
=========================================
  Hits            587      587           
  Misses            9        9           

Files	Coverage Δ
src/DDLQueryBuilder.php	100.00% <100.00%> (ø)

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[vjik]

@Tigrov Add line to changelog only.