replace deprecated getQueryPart() in ContextMapper

[blizzz]

• Doctrine 4 removed getQueryPart() (PR)
• … and marked it as deprecated in 3.8 (PR)
• So it is also marked as deprecated in NCs abstraction (PR) as of NC 30

Money quote:

This function is going to be removed with the next Doctrine/DBAL update and we can not fix this in our wrapper. Please track the details you need, outside the object.

We use it in one place to figure out whether a WHERE condition is already set to know whether we either have to use where() or andWhere() in that method. Cf.

tables/lib/Db/ContextMapper.php

Lines 316 to 320 in 1849033

	if ($qb->getQueryPart('where') === null) {
	$qb->where($whereExpression);
	} else {
	$qb->andWhere($whereExpression);
	}

Server wants to bump to Doctrine 4 in the upcoming version.

• Figure out whether we can solve it with available methods
• If so, implement the changes in a backwards compatible manner
• If not so, bring up necessary changes to the IQueryBuilder abstraction in server and follow up

Side effect: currently there is potential deprecation log spam against >= NC 30.

[blizzz]

From a discussion with nickvergessen,

• andWhere might be possible to use unconditionally, though I believe to remember having had overwrite-issues with this in combination with where(), but need to double check.
• nonEmptyWhere could be a candidate to be exposed per IQueryBuilder otherwise: https://github.com/nextcloud/server/blob/48d69c727ad7cf9c8122743e204a0493584494e4/lib/private/DB/QueryBuilder/QueryBuilder.php#L877-L883