Reproduction steps:
1. Open the attached “BugRepro” project
2. Open the “Assets/Scenes/SampleScene.unity” Scene
3. Select the “Label” GameObject (Canvas > Label) in the Hierarchy window
4. In the Inspector window, in the "Localize String Event" component click the selector dropdown of the "String Reference" field
5. Observe the popup window

Expected result: There is a "None" option
Actual result: There is no "None" option

Reproducible with: 1.5.0-pre.6 (2022.3.41f1, 6000.0.14f1), 1.5.2 (2022.3.41f1, 6000.0.14f1)
Not reproducible with: 1.5.0-pre.5 (2021.3.42f1, 2022.3.41f1, 6000.0.14f1), 1.5.0-pre.6 (2021.3.42f1), 1.5.2 (2021.3.42f1)

Reproducible on: Windows 11
Not reproducible on: No other environment tested