TST: added groupby apply test for nan coerce (#44038)

Author: tyvich

pandas/tests/groupby/test_apply.py

@@ -1157,3 +1157,24 @@ def test_apply_na(dropna):
     result = dfgrp.apply(lambda grp_df: grp_df.nlargest(1, "z"))
     expected = dfgrp.apply(lambda x: x.sort_values("z", ascending=False).head(1))
     tm.assert_frame_equal(result, expected)
+
+
+def test_apply_empty_string_nan_coerce_bug():
+    # GH#24903
+    result = (
+        DataFrame(
+            {
+                "a": [1, 1, 2, 2],
+                "b": ["", "", "", ""],
+                "c": pd.to_datetime([1, 2, 3, 4], unit="s"),
+            }
+        )
+        .groupby(["a", "b"])
+        .apply(lambda df: df.iloc[-1])
+    )
+    expected = DataFrame(
+        [[1, "", pd.to_datetime(2, unit="s")], [2, "", pd.to_datetime(4, unit="s")]],
+        columns=["a", "b", "c"],
+        index=MultiIndex.from_tuples([(1, ""), (2, "")], names=["a", "b"]),
+    )
+    tm.assert_frame_equal(result, expected)