Fix database detection on SQLite with PORTABILITY_EMPTY_TO_NULL

[morozov]

This is primarily a workaround while there are two fundamental issues:

1. The DBAL doesn't support databases on the SQLite platform but there is a wrapper-level method Connection::getDatabase() which is used internally. It cannot be properly implemented in a portable way by definition.
2. When performing its own queries (e.g. database detection or schema introspection), the DBAL should not use user-defined middlewares (because they can be arbitrary) but should use the wrapper-level APIs (because otherwise, it would duplicate the wrapper code). This is what makes us use other hacks like the following across the schema managers' codebase:

   dbal/src/Schema/OracleSchemaManager.php

   Lines 235 to 236 in 279f365

   	$value = array_change_key_case($value, CASE_LOWER);
   	if (! isset($list[$value['constraint_name']])) {

Returning main instead of an empty string as the SQLite database name was originally proposed in #5046.

Fixes #5615.