BUG: concat coercing arrow to object with null type

doc/source/whatsnew/v2.1.0.rst

@@ -471,6 +471,7 @@ Groupby/resample/rolling
 
 Reshaping
 ^^^^^^^^^
+- Bug in :func:`concat` coercing to ``object`` dtype when one column has ``pa.null()`` dtype (:issue:`53702`)
 - Bug in :func:`crosstab` when ``dropna=False`` would not keep ``np.nan`` in the result (:issue:`10772`)
 - Bug in :func:`merge_asof` raising ``KeyError`` for extension dtypes (:issue:`52904`)
 - Bug in :func:`merge_asof` raising ``ValueError`` for data backed by read-only ndarrays (:issue:`53513`)

pandas/core/dtypes/dtypes.py

@@ -2214,10 +2214,13 @@ def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
         # Mirrors BaseMaskedDtype
         from pandas.core.dtypes.cast import find_common_type
 
+        null_dtype = type(self)(pa.null())
+
         new_dtype = find_common_type(
             [
                 dtype.numpy_dtype if isinstance(dtype, ArrowDtype) else dtype
                 for dtype in dtypes
+                if dtype != null_dtype
             ]
         )
         if not isinstance(new_dtype, np.dtype):

pandas/tests/extension/test_arrow.py

@@ -2872,6 +2872,15 @@ def test_conversion_large_dtypes_from_numpy_array(data, arrow_dtype):
     tm.assert_extension_array_equal(result, expected)
 
 
+def test_concat_null_array():
+    df = pd.DataFrame({"a": [None, None]}, dtype=ArrowDtype(pa.null()))
+    df2 = pd.DataFrame({"a": [0, 1]}, dtype="int64[pyarrow]")
+
+    result = pd.concat([df, df2], ignore_index=True)
+    expected = pd.DataFrame({"a": [None, None, 0, 1]}, dtype="int64[pyarrow]")
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize("pa_type", tm.ALL_INT_PYARROW_DTYPES + tm.FLOAT_PYARROW_DTYPES)
 def test_describe_numeric_data(pa_type):
     # GH 52470