Modify default ColumnOption from NOT NULL to NULL

[devgony]

To resolve #974

Goal

For better user experience, Let's modify default ColumnDef.nullable from false to true

CREATE TABLE Items (id INT);

Before

TABLE_NAME	COLUMN_NAME	NULLABLE
Items	id	NOT NULL

After

TABLE_NAME	COLUMN_NAME	NULLABLE
Items	id	NULL

Todo

• modify default ColumnDef.nullable from false to true
• add Column name validation and RowErr::WrongColumnName)

[coveralls]

Pull Request Test Coverage Report for Build 3458000253

• 26 of 26 (100.0%) changed or added relevant lines in 6 files are covered.
• 23 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 98.387%

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Files with Coverage Reduction	New Missed Lines	%
core/src/executor/context/filter_context.rs	4	93.44%
test-suite/src/tester/mod.rs	19	87.5%

Totals
Change from base Build 3451101516:	-0.05%
Covered Lines:	39887
Relevant Lines:	40541

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💛 - Coveralls

[ever0de]

👍

[panarch]

Looks all nice! Thanks a lot 👍