TST: sparse and dense concat test GH30667 (pandas-dev#34355)

Author: ricalanis

pandas/tests/reshape/test_concat.py

@@ -2757,6 +2757,17 @@ def test_concat_sparse():
     tm.assert_frame_equal(result, expected)
 
 
+def test_concat_dense_sparse():
+    # GH 30668
+    a = pd.Series(pd.arrays.SparseArray([1, None]), dtype=np.float)
+    b = pd.Series([1], dtype=np.float)
+    expected = pd.Series(data=[1, None, 1], index=[0, 1, 0]).astype(
+        pd.SparseDtype(np.float64, None)
+    )
+    result = pd.concat([a, b], axis=0)
+    tm.assert_series_equal(result, expected)
+
+
 @pytest.mark.parametrize("test_series", [True, False])
 def test_concat_copy_index(test_series, axis):
     # GH 29879