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

• https://github.com/yiisoft/db-mysql/pull/364
• https://github.com/yiisoft/db-mssql/pull/329
• https://github.com/yiisoft/db-oracle/pull/290
• https://github.com/yiisoft/db-pgsql/pull/371

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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