Add checked methods to add vertex attributes to mesh only if values is not empty

[hukasu]

Objective

When creating a custom mesh, there might be an attribute that is optional, that way you would need to store the Mesh in a variable, insert the required attributes, then check if the optional attribute exists or not, and insert if it does. With this checked methods you can use with the builder patterns

Solution

Create methods insert_attribute_if_not_empty and with_inserted_attribute_if_not_empty (names are tentative).

Testing

Used in a modification to SphereMeshBuilder (PR #17691)

Showcase

Before

        let mut mesh = Mesh::new(
            PrimitiveTopology::TriangleList,
            RenderAssetUsages::default(),
        )
        .with_inserted_indices(indices)
        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, points)
        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs);

         if optional_attribute {
                 let optional_attribute = generate_optional_attribute();
                 mesh.insert_attribute(OPTIONAL_ATTRIBUTE, optional_attribute);
        }

       mesh

After

let optional_attribute = if optional_attribute {
          generate_optional_attribute()
} else {
    vec![]
};

Mesh::new(
    PrimitiveTopology::TriangleList,
    RenderAssetUsages::default(),
)
.with_inserted_indices(indices)
.with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, points)
.with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
.with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
.with_inserted_attribute_if_not_empty(OPTIONAL_ATTRIBUTE, optional_attribue)