title: Return Empty column if column does not exist in dataframe
body: <p>I created a dataframe <code>df</code> as below:</p>
  
  <pre><code>Type = ['A', 'B', 'C', 'D']
  Size = [72,23,66,12]
  df = pd.DataFrame({'Type': Type, 'Size': Size})
  </code></pre>
  
  <p>I can extract any existing column using:</p>
  
  <pre><code>df_count = df['Size']
  </code></pre>
  
  <p>However, if a data frame is too big, and I don't know if the column exist in df or not. In such if I call a column e.g. <code>df['Shape']</code> as below:</p>
  
  <pre><code>df_null = df['Shape']
  </code></pre>
  
  <p>It raises a <code>KeyError</code>. However I want that <code>df_null</code> should get an empty column with name <code>"Shape"</code>.</p>