Fix CAST/CONVERT of NULL returning 0, false or "undefined" to close #1208

[joshuablac]

Closes #1208

Problem

CAST/CONVERT coerce NULL instead of passing it through, so AVG(CAST(col AS NUMBER)) over NULL values returns 0 instead of undefined (the example in #1208). The same root cause shows up for every target type:

Expression	develop	With this PR
CAST(NULL AS INT) / SMALLINT / BIGINT	0	undefined
CAST(a AS NUMBER) where a is null	0	undefined
CAST(NULL AS VARCHAR)	"undefined" ("null" for a null column)	undefined
CAST(NULL AS BOOLEAN)	false	undefined
CAST(NULL AS DATE)	"aN.aN.aN" ("70.01.01" for a null column)	undefined
CAST(NULL AS JSON)	throws Cannot convert string to JSON	undefined
AVG / SUM / COUNT(CAST(a AS NUMBER)) over [null, 4, missing]	1.33 / 4 / 3	4 / 4 / 1

Cause

alasql.stdfn.CONVERT (used by both CAST and CONVERT) never checks for NULL, so null goes through val | 0, +val, String(val), !!val or the date formatting.

Fix

Return undefined at the top of CONVERT when the value is null or undefined. That matches standard SQL (casting NULL to any type gives NULL) and AlaSQL's convention of returning undefined for NULL. Non-null values are converted as before.

Behavior change to note: anything that relied on CAST(NULL AS INT) being 0 (or BOOLEAN being false, etc.) now gets undefined. The same applies to DECLARE/SET on a typed variable assigned NULL, since they also go through CONVERT. No existing test depended on the old behavior.

Tests

• Added test/test1208.js: both queries from the issue, CAST(NULL AS ...) for 17 types, null and missing columns, CONVERT with a style, aggregates over cast NULLs, and a guard that non-null values still convert. 6 of the 7 cases fail on develop and all 7 pass with this change.
• Full suite (bun test, same command as yarn test-only): 2256 pass, 213 skip. The only 2 failures are in test2149.js (CLI). They fail identically on unmodified develop in my local setup: one because my checkout path contains spaces, and one because the version-injection step of build.sh didn't run.

Note: while testing I noticed that plain AVG(col) (without CAST) returns null for a single-row group whose value is null, but undefined for a multi-row all-null group. This PR doesn't touch that.

This fix was written with AI assistance (Claude Code) and verified locally as described above.

🤖 Generated with Claude Code

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