title: Check if Pandas Series is of type string
body: <p>I import some data from a parquet file into a DataFrame and want to check the data types. One of the data types I expect is strings. To do this, I have something like the following:</p>
  
  <pre><code>import pandas as pd
  col = pd.Series([None, 'b', 'c', None, 'e'])
  assert((col.dtype == object) and (isinstance(col[0], str)))
  </code></pre>
  
  <p>But, as you can see, this does not work if I accidentally have a <code>None</code> value at the beginning.</p>
  
  <p>Does anybody have an idea how to do that efficiently (preferably without having to check each element of the series)?</p>