Notify changes on calling `ArrayMesh.clear_surfaces()`

[aoisensi]

When calling ArrayMesh.clear_surfaces() while the Inspector is open, and then leaving the mesh without adding any new surfaces, the Godot Editor begins to continuously spam errors. This commit fixes that issue.

Reproduction code: https://gist.github.com/aoisensi/f544d06602cafca7f915c777459c653e

The sample lets you increase or decrease the number of surfaces using a button. If you reduce the surface count to zero, the bug described above will occur.