fix(ddl): fix modify column data type null to not null failed

[b41sh]

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Summary

1. fix modify column data type null to not null failed, using remove_nullable function to remove null value from the original column.
2. nullable random column values remove unused underlaying values.

• fixes: bug: Can't read table by alter table column from string null to string not null #17648

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[wubx]

LGTM
I've test this pr.