Existing Item Quantities at (default)Source set to zero when saving product images with gallery management

### Preconditions

1. Magento 2.3
2. MSI 1.1.0
3. php 7.2
4. single store mode ("default" source only)
### Steps to reproduce

1. create new simple product with ProductInterfaceFactory
2. save with product repository (no stock data set yet)
3. save inventory to "default" source with:
Magento\InventoryApi\Api\SourceItemsSaveInterface
4. inventory successfully saves
5. add images to product using:
'Magento\Catalog\Model\Product\Gallery\GalleryManagement'
6. Product inventory for "default" source, gets set to ZERO.

This was super freaking tedious to debug.

If you save images FIRST with gallery management, and then set stock values after, stock quantities will not be set to 0 and things work as expected.

### Expected result

1.

### Actual result

1. [Screenshots, logs or description]
2.

Reproduce the issue with ProductInterfaceFactory, SourceItemsSaveInterface, and Product\Gallery\GalleryManagement in the Magento 2.3/MSI 1.1.0 setup described. Trace the product save and source-item handling after gallery management is called, then verify that an existing default-source quantity remains unchanged when images are added.

Written by the indexing model from the issue text.