fix: nil database/sql arg produces NullType instead of Void

[Copilot]

Passing nil as a database/sql argument was converted to YDB Void, which YDB rejects for optional columns. It should produce NullType instead.

db.ExecContext(ctx, "UPSERT INTO tbl (id, val) VALUES ($id, $val)",
    sql.Named("id", int64(1)),
    sql.Named("val", nil), // was: Void → now: NullType
)

Changes

• internal/bind/params.go: toValue() nil case now returns value.NullValue(types.Null{}) instead of value.VoidValue()
• internal/bind/params_test.go: Updated expected value for nil input accordingly
• tests/integration/database_sql_nil_arg_test.go: Integration test — creates a table with an optional column, upserts a row with nil, reads back and asserts NULL

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>bug: nil as database/sql arg transforms to Void, but must be a NullType</issue_title>
<issue_description>```go
result, err := db.ExecQuery(ctx, "INSERT INTO tbl (a, b, c) VALUES ($p1, $p2, $p3)",
sql.Named("p1", 1),
sql.Named("p2", nil), // produced Void
sql.Named("p3", 3),
)

YDB not accepts [Void](https://github.com/ydb-platform/ydb-api-protos/blob/7aa20b63d6cb2bd856ff0562d8b275e65b76f83b/protos/ydb_value.proto#L118) value for insert into Optional column
Special type [NullType](https://github.com/ydb-platform/ydb-api-protos/blob/7aa20b63d6cb2bd856ff0562d8b275e65b76f83b/protos/ydb_value.proto#L119) helps to accept nil from user and apply to YDB on INSERT queries</issue_description>

<agent_instructions>Переделай обработку nil (сейчас Void, должно быть NUllType) в качестве аргументов database/sql
Добавь юнит-тесты и интеграционные тесты (создание таблицы, вставка строки)</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes bug: nil as database/sql arg transforms to Void, but must be a NullType #2062

---

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[asmyasnikov]

@copilot откати все изменения в go.mod

[github-actions]

summary

Inferred base version: v3.135.3
Suggested version: v3.135.4

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.05%. Comparing base (d892a39) to head (e6b6a23).
⚠️ Report is 32 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (d892a39) and HEAD (e6b6a23). Click for more details.

HEAD has 465 uploads less than BASE

Flag	BASE (d892a39)	HEAD (e6b6a23)
macOS	21	2
unit	65	6
go-1.26.x	72	3
go-1.21.x	65	3
windows	22	2
ubuntu	94	2
integration	63	0
ydb-24.4	22	0
ydb-latest	21	0
experiment	9	0
ydb-nightly	9	0
ydb-edge	20	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2063       +/-   ##
===========================================
- Coverage   75.05%   46.05%   -29.00%     
===========================================
  Files         427      426        -1     
  Lines       36902    36767      -135     
===========================================
- Hits        27696    16933    -10763     
- Misses       8025    19206    +11181     
+ Partials     1181      628      -553     

Flag	Coverage Δ
experiment	?
go-1.21.x	45.99% <100.00%> (-26.58%)	⬇️
go-1.26.x	46.04% <100.00%> (-28.87%)	⬇️
integration	?
macOS	46.04% <100.00%> (-0.03%)	⬇️
ubuntu	46.04% <100.00%> (-29.01%)	⬇️
unit	46.05% <100.00%> (-0.07%)	⬇️
windows	46.03% <100.00%> (-0.05%)	⬇️
ydb-24.4	?
ydb-edge	?
ydb-latest	?
ydb-nightly	?

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[Copilot]

@copilot откати все изменения в go.mod

Done in bc1fea8 — reverted examples/go.sum, tests/slo/go.mod, and tests/slo/go.sum back to their original state.