Consider two empty matrices as equal.

src/base/matrix.rs

@@ -894,10 +894,14 @@ where
 {
     #[inline]
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        if self.shape() != other.shape() || self.nrows() == 0 || self.ncols() == 0 {
+        if self.shape() != other.shape() {
             return None;
         }
 
+        if self.nrows() == 0 || self.ncols() == 0 {
+            return Some(Ordering::Equal);
+        }
+
         let mut first_ord = unsafe {
             self.data
                 .get_unchecked_linear(0)