title: Rename "None" value in Pandas
body: <p>This is probably super simple but I just can not find the answer. I import data using GeoPandas from a shape file. Turn that into pandas <code>DataFrame</code>. I have a object field with three letter codes and <code>None</code> values for missing data. How do I change <code>None</code>'s to something like "vcv" in pandas? I tried this</p>
  
  <pre><code>sala.replace(None,"vcv")
  </code></pre>
  
  <p>got this error</p>
  
  <pre><code>   2400                                     "strings or regular expressions, you "
     2401                                     "passed a"
  -&gt; 2402                                     " {0!r}".format(type(regex).__name__))
     2403                 return self.replace(regex, value, inplace=inplace, limit=limit,
     2404                                     regex=True)
  
  TypeError: 'regex' must be a string or a compiled regular expression or a list or dict of strings or regular expressions, you passed a 'bool'
  </code></pre>
  
  <p>Tried this</p>
  
  <pre><code>if sala['N10'] is None:
      sala['N10'] = 'Nul'
  </code></pre>
  
  <p>Does not change anything. </p>