fix for infering dtype in failing s.combine on extention array

Author: vladserkoff

pandas/core/series.py

@@ -18,6 +18,7 @@
 from pandas.util._decorators import Appender, Substitution, deprecate
 from pandas.util._validators import validate_bool_kwarg
 
+from pandas.core.dtypes.cast import infer_dtype_from
 from pandas.core.dtypes.common import (
     _is_unorderable_exception, ensure_platform_int, is_bool,
     is_categorical_dtype, is_datetime64_dtype, is_datetimelike, is_dict_like,
@@ -2648,7 +2649,10 @@ def combine(self, other, func, fill_value=None):
             # The function can return something of any type, so check
             # if the type is compatible with the calling EA.
             non_na_values = [v for v in new_values if notna(v)]
-            new_dtype, _ = infer_dtype_from(non_na_values, pandas_dtype=True)
+            if non_na_values:
+                new_dtype, _ = infer_dtype_from(non_na_values, pandas_dtype=True)
+            else:
+                new_dtype = self.dtype
             try:
                 new_values = self._values._from_sequence(new_values, dtype=new_dtype)
             except Exception: