title: Merging two dataframes with pd.NA in merge column yields 'TypeError: boolean value of NA is ambiguous'
body: <p>With Pandas 1.0.1, I'm unable to merge if the </p>
  
  <pre><code>df = df.merge(df2, on=some_column)
  </code></pre>
  
  <p>yields</p>
  
  <pre><code>File /home/torstein/code/fintechdb/Sheets/sheets/gild.py, line 42, in gild
      df = df.merge(df2, on=some_column)
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/frame.py, line 7297, in merge
      validate=validate,
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 88, in merge
      return op.get_result()
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 643, in get_result
      join_index, left_indexer, right_indexer = self._get_join_info()
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 862, in _get_join_info
      (left_indexer, right_indexer) = self._get_join_indexers()
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 841, in _get_join_indexers
      self.left_join_keys, self.right_join_keys, sort=self.sort, how=self.how
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 1311, in _get_join_indexers
      zipped = zip(*mapped)
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 1309, in &lt;genexpr&gt;
      for n in range(len(left_keys))
  File /home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/reshape/merge.py, line 1918, in _factorize_keys
      rlab = rizer.factorize(rk)
  File pandas/_libs/hashtable.pyx, line 77, in pandas._libs.hashtable.Factorizer.factorize
  File pandas/_libs/hashtable_class_helper.pxi, line 1817, in pandas._libs.hashtable.PyObjectHashTable.get_labels
  File pandas/_libs/hashtable_class_helper.pxi, line 1732, in pandas._libs.hashtable.PyObjectHashTable._unique
  File pandas/_libs/missing.pyx, line 360, in pandas._libs.missing.NAType.__bool__
  
  TypeError: boolean value of NA is ambiguous
  </code></pre>
  
  <p>while this works:</p>
  
  <pre><code>df[some_column].fillna(np.nan, inplace=True)
  df2[some_column].fillna(np.nan, inplace=True)
  df = df.merge(df2, on=some_column)
  # Works
  </code></pre>
  
  <p>If instead, I do</p>
  
  <pre><code>df[some_column].fillna(pd.NA, inplace=True)
  </code></pre>
  
  <p>then the error returns.</p>