[fupelaqu]

Summary

• CAST(NULL AS VARCHAR) answers NULL instead of the text 'null'. A NULL literal converted to any type now renders null: the literal is never nullable, so the VARCHAR arm wrapped it unguarded in String.valueOf(null).
• A set-operation leg's slot builds its reader once instead of twice (arrow's plan-time typing for set operations, SOFTNETWORK-APP/softclient4es-arrow#198).

Behaviour (measured on Elasticsearch 8.18.3 through the gateway)

statement	0.23.0	main	this PR
SELECT CAST(NULL AS VARCHAR) AS sv FROM t	NULL	text 'null' (regression)	NULL
SELECT n, CAST(NULL AS VARCHAR) AS sv, CAST(NULL AS INT) AS iv FROM t	sv = text 'null'	text 'null'	NULL
CAST(NULL AS INT) (control)	NULL	NULL	NULL

Every other cast is unchanged; set-operation answers are unchanged (the reader change only stops building the same reader twice).

Tests

• CastConversionSpec: a NULL cast to VARCHAR, CHAR, TEXT, KEYWORD, INT, BIGINT, DOUBLE, REAL, BOOLEAN, DATE, TIME, TIMESTAMP and DATETIME renders null, alone and beside other columns; it failed on main with String.valueOf(null).
• DottedColumnSpec (shared testkit, every major's runner): CAST(NULL AS VARCHAR) answers NULL on every row route, alone and beside other columns.
• Local runs: sql/test 1,847 / 1,847, core/test 1,259 / 1,259, es8java/test green (the two first-run failures were the new test's REPL harness, since moved, and a fresh-worktree resource ordering artefact that passes on rerun).
• Downstream, built against this core published locally:
  • softclient4es-extensions: community/test 223 / 223 and its ES 8 views suite 20 / 20, with no change needed.
  • softclient4es-arrow (SOFTNETWORK-APP/softclient4es-arrow#198, set-operation typing on core's leg schemas): unit suites green, ES 8 ADBC 78 / 78 and Flight 249 / 249, ES 6.8 perf(core): guard tryParseAsDateTime with a cheap temporal-shape check (arrow#139) #198 blocks 12 / 12 and 11 / 11, the Python Flight job 31 / 31, + compile, and its regression gate against arrow 264eac3 on the same jars: 0 violations. Its new test of the CAST(NULL AS VARCHAR) workaround needs this core.
• CI's lint line passes on a clone of the committed bytes.

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