fix: return Int64 from null_count on SQL backends, and 0 on empty frames

[FBruzzesi]

Description

DuckDB widens integer sum/count_if to HUGEINT, so null_count returned Int128/Decimal(38, 0) on DuckDB and SQLFrame (where collect_schema() also raised), and NULL instead of 0 on empty frames on DuckDB, SQLFrame and Ibis.
A single SQLExpr.null_count (coalesced sum, cast to Int64 outside any over) replaces the three backend implementations, with no slowdown in select, group_by or over, and about half the memory when collecting over results.

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes [Bug]: null_count() is not an integer dtype on the DuckDB backend, unlike count() and len() #4024

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