Problem/Motivation
Deprecate Drupal\Core\Database\StatementEmpty as it only is used in testing and therefore does not belong in the main namespace.
Proposed resolution
Deprecate Drupal\Core\Database\StatementEmpty and replace where it is used. For pure testing, StatementInterface could be mocked to return empty values.
Remaining tasks
TBD
User interface changes
None
API changes
The class Drupal\Core\Database\StatementEmpty will be deprecated.
Data model changes
None
Release notes snippet
TBD
