Fix ColumnDefinitionParser to parse unquoted DEFAULT NULL as SQL NULL

[Lugat]

As discussed in yiisoft/db-mysql#485: an unquoted NULL after DEFAULT was stored as defaultValueRaw = 'NULL'. The MySQL ColumnFactory typecasts it by column type, so int DEFAULT NULL was created as DEFAULT 0. This breaks the yiisoft/queue-db migration: messages are never reserved because reserved_at IS NULL never matches.

String defaults are always quoted in a column definition, so an unquoted NULL can only be SQL NULL. It is now parsed as defaultValueRaw = null. Schema reading from information_schema is not affected, so varchar DEFAULT 'NULL' keeps working.

Checked on MySQL 8.0: int DEFAULT NULL → DEFAULT NULL (was DEFAULT 0); int DEFAULT 5 and varchar(10) DEFAULT 'NULL' are unchanged.

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	yiisoft/db-mysql#485

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.76%. Comparing base (2d126ae) to head (fc4fece).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1202      +/-   ##
============================================
+ Coverage     98.63%   98.76%   +0.13%     
- Complexity     1653     1655       +2     
============================================
  Files           123      123              
  Lines          4309     4305       -4     
============================================
+ Hits           4250     4252       +2     
+ Misses           59       53       -6     

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[Tigrov]

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