All physics simulation and queries stop working, with no message in the Console, when the Editor is launched with an unrecognized or empty -physicsBackend value

Steps to reproduce:

1. Create a new 3D URP project
2. In the Hierarchy, right-click > 3D Object > Cube
3. Select the Cube and in the Inspector Add Component > Rigidbody
4. Enter Play mode and observe the Cube falls
5. Close the Editor
6. Click on the projects kebab (⋮) menu and choose "Add command line argument" (or run it from terminal)
7. Add -physicsBackend and save
8. Open the project and enter Play mode
9. Observe the Cube

Actual results: Cube does not move/fall and no messages appear in the Console

Expected results: Either a warning/error is logged or physics work

Reproducible with versions: 6000.3.24f1, 6000.6.2f1, 6000.7.0b2

Tested on (OS): macOS 26.5, Ubuntu 24.04

Notes:

• Also reproduces with an unrecognized name (-physicsBackend TotallyNotARealBackend) and with an explicitly empty value (-physicsBackend "")
• Does not reproduce with -physicsBackend PhysX or -physicsBackend physx - both select PhysX normally, so name matching is case-insensitive
• Does not reproduce in 6.0 and 6.2, but likely not a regression but the feature was not present
• Physics queries fail as well as simulation: Physics.Raycast returns false against a collider directly in the ray's path, Physics.OverlapSphere returns 0, Physics.CheckSphere returns false
• Physics.defaultPhysicsScene.IsValid() still returns true, so the state is not detectable from script