Fix GROUP_CONCAT to handle NULL values per SQL semantics

[Copilot]

GROUP_CONCAT was converting NULL values to the string "null" instead of skipping them, causing incorrect concatenation results like "x,null,z" and failing with DISTINCT.

Changes

src/55functions.js

• Skip NULL/undefined values during concatenation stages
• Return NULL when all aggregated values are NULL
• Handle accumulator initialization when first value is NULL

test/test942.js

• Validate NULL skipping in concatenation
• Validate NULL return for all-NULL groups
• Validate DISTINCT with NULLs
• Cover grouped and non-grouped queries

Before/After

// Before
GROUP_CONCAT(b) with ['x', null, 'z'] → "x,null,z"
GROUP_CONCAT(b) with [null, null]     → "null,null"
GROUP_CONCAT(DISTINCT b) with ['x', null, 'x', null] → "x,null"

// After
GROUP_CONCAT(b) with ['x', null, 'z'] → "x,z"
GROUP_CONCAT(b) with [null, null]     → null
GROUP_CONCAT(DISTINCT b) with ['x', null, 'x', null] → "x"

Original prompt

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This section details on the original issue you should resolve

<issue_title>GROUP_CONCAT is treating null as "null"</issue_title>
<issue_description>When using GROUP_CONCAT null values are grouped like strings, for example it will group 2 rows with null into "null,null".

When used with DISTINCT it will return a single "null" instead of a JS null.

If I remember / have time I'll come back and see if I can easily make a PR for this, but just wanted to document the issue for now.
</issue_description>

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

@mathiasrw Thank you so much for looking into this. Please let me know if you find out something.

• Fixes GROUP_CONCAT is treating null as "null" #1259

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