code: def test_combine_first_string_dtype_only_na(
      self, nullable_string_dtype
  ):
      # GH: 37519
      df = DataFrame(
          {"a": ["962", "85"], "b": [pd.NA] * 2},
          dtype=nullable_string_dtype,
      )
      df2 = DataFrame(
          {"a": ["85"], "b": [pd.NA]}, dtype=nullable_string_dtype
      )
      df.set_index(["a", "b"], inplace=True)
      df2.set_index(["a", "b"], inplace=True)
      result = df.combine_first(df2)
      expected = DataFrame(
          {"a": ["962", "85"], "b": [pd.NA] * 2},
          dtype=nullable_string_dtype,
      ).set_index(["a", "b"])
      tm.assert_frame_equal(result, expected)
  
imports: from datetime import datetime
  import numpy
  import pytest
  from pandas.core.dtypes.cast import find_common_type
  from pandas.core.dtypes.common import is_dtype_equal
  import pandas
  from pandas import DataFrame, Index, MultiIndex, Series
  import pandas._testing
label_desc: Use image node_id 18 for calling the TestDataFrameCombineFirst obj's underlying member method code with example usage: obj.test_combine_first_string_dtype_only_na(nullable_string_dtype) without return types