[AthenaConfiguration] conf2toConfigurable: Allow merging conf2 private tool into conf1 empty private tool handle

A tiny yet crucial fix in TrigOutputHandling and a moderately significant change in AthenaConfiguration to allow that fix. The changes are two separate commits.

AthenaConfiguration:

When converting GaudiConfig2 configurables into old-config, allow a conf2 private tool handle which holds a tool to be merged into conf1 tool handle which is empty. This seems natural and reflects the code that is already in place for private and public tool handle arrays. It just wasn't done for simple tool handles. The implementation is mostly copied from the private tool handle array solution a few lines above.

TrigOutputHandling (use case for the above):

HLTResultMTMaker is a tool always required by the HLT framework and configured in the base config even for the simplest HLT jobs like HelloWorld. In full HLT operation the tool uses sub-tools which depend on the trigger menu and thus require extensive trigger configuration. The menu and these sub-tools should be absent in simple jobs like HelloWorld. Therefore, these sub-tool handles need to be empty by default and filled only by job config fragments which also configure the menu. In this case merging of these tools into the empty tool handles is required.

Running full-unit-tests because of the AthenaConfiguration changes.

cc @tbold

Fixes ATR-23514

RC: Please do not squash the commits.