title: Column having no values gives the error 'can not infer schema' while reading excel to dataframe using koalas read_excel()
body: <p>While reading excel file as dataframe using databricks koalas read_excel() with dtype as <strong>str</strong>, if a column is not having values it gives the error</p>
  <blockquote>
  <p>can not infer schema from empty dataset</p>
  </blockquote>
  <p>How to solve this issue?
  If I change the dtype to <strong>None</strong>, it will not throw error. But numeric data will be read in scientific form.</p>
  <p>I tried writing converter:</p>
  <pre><code>converters={i : (lambda x: str(x) if x or x!='' else np.NaN) for i in range(col_count)}
  </code></pre>
  <p>(dtype=str is not working with converter, so removed).
  But this will read the string 'NA' as null.
  I want the data as it is in the source file.</p>