Improve column schema classes

[Tigrov]

• Improve psalm annotations;
• add ColumnSchemaInterface::null() method as alias to ColumnSchemaInterface::notNull(false)
  to be compatible with ColumnInterface;
• add ColumnSchemaInterface::hasDefaultValue() method to check if default value is initialized, even if it is null value.

Related PRs

• Improve column schema classes db-mysql#364
• Improve column schema classes db-mssql#329
• Improve column schema classes db-oracle#290
• Improve column schema classes db-pgsql#371

Q	A
Is bugfix?	❌
New feature?	✔️/❌
Breaks BC?	❌
Fixed issues	#854

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (86fc92e) to head (aa7e304).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #899   +/-   ##
=========================================
  Coverage     99.60%   99.60%           
- Complexity     1538     1540    +2     
=========================================
  Files            81       81           
  Lines          3789     3801   +12     
=========================================
+ Hits           3774     3786   +12     
  Misses           15       15           

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