fix(drizzle, db-mongodb): handle ['null'] array when filtering hasMany relationship field by None

[lcnogueira]

What?

When filtering a hasMany relationship field by None in the Payload admin panel list view, no results are returned — even when documents with no related entries exist.

Why?

The issue exists in both packages/drizzle/src/queries/sanitizeQueryValue.ts and packages/db-mongodb/src/queries/sanitizeQueryValue.ts, inside the relationship/upload type block.

When the admin UI sends a "None" filter for a hasMany relationship field, the value arrives as ['null'] — an array containing the string 'null'. The current null check in both adapters only handles the plain string form:

if (val === 'null') {
  formattedValue = null
}

Since ['null'] !== 'null', the check is bypassed. In the drizzle adapter, the ['null'] value then reaches the array-handling branch which converts operator: 'equals' to operator: 'in' with value: ['null'], generating:

WHERE types_id IN ('null')

In the MongoDB adapter, the unconverted ['null'] value causes a query error, returning { errors: [...] } instead of results.

Both cases result in the filter producing no valid output.

How?

Extended the null check in both adapters to also handle the ['null'] array form that hasMany relationship fields produce:

// before
if (val === 'null') {
  formattedValue = null
}

// after
if (val === 'null' || (Array.isArray(val) && val.length === 1 && val[0] === 'null')) {
  formattedValue = null
}

Fixes #16292