A ShadowCaster2D with no shape source leaks one Mesh per instance when created and destroyed in the Editor (unbounded)

Steps to reproduce:

1. Open any URP project that has the 2D Renderer / URP 2D runtime
2. Put the attached ShadowCaster2DLeakTest.cs in an Assets/Editor folder and run Bug > ShadowCaster2D Shapeless Leak Test. It repeatedly creates 25 ShadowCaster2D components that have NO shape source (no Collider2D, no SpriteRenderer, no shapePath), destroys them, and measures the loaded Mesh count between cycles after GC + Resources.UnloadUnusedAssets (run twice).

Actual results: The loaded Mesh count grows per shapeless ShadowCaster2D every cycle and is never reclaimed - an unbounded leak. GC + Resources.UnloadUnusedAssets (run twice) do not free them. Each leaked object is a small ShadowCaster2D shadow mesh (3-4 vertices, hideFlags None). The baseline keeps climbing for as long as shapeless casters are created and destroyed in the session.

Expected results: Destroying a ShadowCaster2D should release the shadow Mesh it created, regardless of whether the caster had a shape source, so the loaded Mesh count returns to baseline. No unbounded growth.

Reproducible with versions: 6000.0.0f1, 6000.0.84f1, 6000.3.24f1, 6000.6.1f1, 6000.7.0b1, 7000.0.0a1

Tested on (OS): Windows 11