fix: use AND instead of OR in neq filter for null-equivalent values (#19071)

Fixes #19070

The `neq` operator in `compute-where-condition-parts.ts` uses `OR` where
it should use `AND` when handling null-equivalent values.

Currently generates:
```sql
field != '' OR field IS NOT NULL
```

For a row where `field = ''`:
- `'' != ''` = false
- `'' IS NOT NULL` = true
- `false OR true` = true -- row incorrectly passes the filter

The `eq` operator correctly uses `OR field IS NULL` because it's
additive (match value or its null equivalent). By De Morgan's law, the
negation `neq` needs `AND field IS NOT NULL` -- exclude if the value
doesn't match AND is not a null equivalent.

With the fix:
```sql
field != '' AND field IS NOT NULL
```
- `'' != ''` = false, `'' IS NOT NULL` = true, `false AND true` = false
-- correctly excluded
- `NULL != ''` = NULL, `NULL IS NOT NULL` = false, `NULL AND false` =
false -- correctly excluded
- `'Alice' != ''` = true, `'Alice' IS NOT NULL` = true, `true AND true`
= true -- correctly included

Affects `neq` filters on TEXT fields and all composite sub-fields
(firstName, lastName, primaryEmail, primaryPhoneNumber, address
sub-fields, etc.) when filtering against null-equivalent values.

Co-authored-by: Etienne <45695613+etiennejouan@users.noreply.github.com>

packages/twenty-server/src/engine/api/graphql/graphql-query-runner/utils/compute-where-condition-parts.ts

@@ -65,7 +65,7 @@ export const computeWhereConditionParts = ({
       };
     case 'neq':
       return {
-        sql: `${fieldReference} != :${key}${paramSuffix}${hasNullEquivalentFieldValue ? ` OR ${fieldReference} IS NOT NULL` : ''}`,
+        sql: `${fieldReference} != :${key}${paramSuffix}${hasNullEquivalentFieldValue ? ` AND ${fieldReference} IS NOT NULL` : ''}`,
         params: { [`${key}${paramSuffix}`]: value },
       };
     case 'gt':