fix: Copy height in .vstack() for empty dataframes (#19641) (#19642)

crates/polars-core/src/frame/mod.rs

@@ -1084,6 +1084,7 @@ impl DataFrame {
                 self.width(), other.width(),
             );
             self.columns.clone_from(&other.columns);
+            self.height = other.height;
             return Ok(self);
         }
 
@@ -3567,6 +3568,21 @@ mod test {
         assert_eq!(df.first_col_n_chunks(), 2)
     }
 
+    #[test]
+    fn test_vstack_on_empty_dataframe() {
+        let mut df = DataFrame::empty();
+
+        let df_data = df! {
+            "flt" => [1., 1., 2., 2., 3., 3.],
+            "int" => [1, 1, 2, 2, 3, 3, ],
+            "str" => ["a", "a", "b", "b", "c", "c"]
+        }
+        .unwrap();
+
+        df.vstack_mut(&df_data).unwrap();
+        assert_eq!(df.height, 6)
+    }
+
     #[test]
     #[cfg(feature = "zip_with")]
     #[cfg_attr(miri, ignore)]