feat: add is_nullable scalar UDF

[adriangb]

Summary

• Adds is_nullable(expression) scalar UDF that returns the schema-level nullability of the input expression as a boolean
• Completes the Arrow field introspection set alongside arrow_typeof and arrow_metadata
• Returns true for nullable columns/expressions, false for non-nullable ones (NOT NULL columns, concrete literals)

Test plan

• Added sqllogictest (is_nullable.slt) covering:
  • Literal expressions (non-nullable: is_nullable(1) → false)
  • NULL literal (nullable: is_nullable(NULL) → true)
  • Nullable columns (id, name from table_with_metadata)
  • Non-nullable columns (ts, nonnull_name from table_with_metadata)
  • Expression nullability propagation (id + 1 inherits nullable)
• cargo fmt --all passes
• cargo clippy passes
• Regenerated scalar_functions.md docs

🤖 Generated with Claude Code

[adriangb]

@martin-g another one around introspection. Up to you but one option would be to close this PR and say "use arrow_field(...)['nullable']

[martin-g]

"use arrow_field(...)['nullable']"

I like it!

[adriangb]

Okay lets focus on #21389 for now then