Dragging an obs with an ID onto an obs without one should create an obs with an ID

[kueda]

Describe the bug
In the Uploader, dragging an observation with an identification onto an observation without an identification leaves an observation without an identification. It should use the identification from the card being dragged if the card it's being dragged on to has no identification

To Reproduce
Steps to reproduce the behavior:

1. Go to https://www.inaturalist.org/observations/upload
2. Create an observation without media and add an ID
3. Create another observation without media and don't add an ID
4. Drag the observation with an ID onto the observation without an ID: the resulting observation will have no ID

Expected behavior
When dragging an observation with an ID onto an observation without an ID, the resulting combined observation should have the ID of the observation that was dragged. However, if both observations have IDs, the ID of the observation that is being dragged on to should be preserved.

Context

• OS: Mac OS 10.13.6
• Browser: Brave Version 1.27.111 Chromium: 92.0.4515.131 (Official Build) (x86_64)