Fix import and saving related crashes

[AThousandShips]

• Don't add empty mesh to result when importing obj files
• Check for null resources in ResourceSaver

This adds the check from core_bind to ResourceSaver directly, could remove the one in ResourceSaver.save in core_bind as it's now redundant, but just added it for now, it helps catch broken cases when doing things from c++

The obj side avoids doing any of the remaining code, makes things a bit clearer, and matches the code here:

godot/editor/import/3d/resource_importer_obj.cpp

Lines 441 to 445 in fe01776

	if (mesh->get_surface_count() > 0) {
	mesh->set_name(name);
	r_meshes.push_back(mesh);
	mesh.instantiate();
	}

For this specific case we might want to look at adding support for lines in obj files as they support it, or a more clear error message or warning when using unsupported features, but that's outside the scope of this

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• Fixes: Importing a line only obj crash Godot Editor #89776

[akien-mga]

This adds the check from core_bind to ResourceSaver directly, could remove the one in ResourceSaver.save in core_bind as it's now redundant, but just added it for now, it helps catch broken cases when doing things from c++

I would remove the redundant core_bind checks then, as it seems to be wasteful for performance to do these checks twice.

[AThousandShips]

Done!

[AThousandShips]

This one isn't entirely redundant as it exits a bit faster, but it's only skipping over the loop which is minimal

[akien-mga]

Thanks!

[AThousandShips]

Thank you!