compiler/rust: replace !first.is_none() with first.is_some()

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38807>

src/compiler/rust/proc/as_slice.rs

@@ -114,7 +114,7 @@ pub fn derive_as_slice(
                             f_attr.is_none(),
                             "{attr_name} attribute is only allowed on {slice_type}"
                         );
-                        if !first.is_none() {
+                        if first.is_some() {
                             found_last = true;
                         }
                     }