handle NULL VARBINARY values correctly in ConvertValue

[Mandukhai-Alimaa]

NULL VARBINARY values were incorrectly being converted to empty []byte{} instead of nil.

The root cause was that scanNullBytes() returned []byte directly, which when combined with Go's driver.Value interface conversion, lost the NULL semantics in certain scanning scenarios. (The issue was []byte(nil) getting converted to []byte{} through interface{} conversions.)

This change implements a proper NullBinary type following the same pattern as sql.NullString, sql.NullInt64, etc., ensuring consistent NULL handling:

• Add NullBinary struct with Bytes []byte and Valid bool fields
• Update scanNullBytes() to return NullBinary instead of []byte
• Update ConvertValue for "varbinary" case to use NullBinary pattern

NULL values now properly return nil, empty values return []byte{}

Closes #166

[cla-bot]

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[Mandukhai-Alimaa]

Hi @nineinchnick, I sent the contributor license agreement 5 days ago. How do we know if it was successful or not? Also, is there anything else I need to do before this PR can be merged? Thanks so much.

[nineinchnick]

CLA submissions are processed manually by Martin every two or three weeks. The last batch was done about two weeks ago. You don't have to do anything, I keep track of this, and will merge this PR once the CLA is processed.