title: replacing null values in a Pandas Dataframe using applymap
body: <p>I've got an "Age" column, but sometimes NaN values are displayed. 
  I know I can use "fillna" for this purposes but I've tried to define my own function (and learning to do this way) and use applymap to dataframe</p>
  
  <p>no success so far.</p>
  
  <pre><code>Age
  69
  49
  NaN
  54
  NaN
  </code></pre>
  
  <p>I've tried</p>
  
  <pre><code>   def get_rid_of_nulls(value):
       if value == np.nan:
          return 'Is Null value'
       else:
          return value
  </code></pre>
  
  <p>with this not working either</p>
  
  <pre><code> if value == None
     if value isnull
     if value == np.na
     if value ==''
     if value == NaN
     if value == 'NaN'
  </code></pre>
  
  <p>None of the comparisons seems to work. I'm wrong for sure but I'm stuck and I'm very stubborn to use fillna</p>
  
  <p>thanks</p>