text: Title: concatenate_columns adds the function of ignoring empty values
  Body: # Problem description
  
  When I use the function of concatenating multiple columns, I find that it does not handle null values as expected.
  
  ## This is the current output
  
  ```Python
  df.concatenate_columns(["cat_1","cat_2","cat_3"],"cat",sep=",")
  ```
  
  <table border="1" class="dataframe">  <thead>    <tr style="text-align: right;">      <th></th>      <th>cat_1</th>      <th>cat_2</th>      <th>cat_3</th>      <th>cat</th>    </tr>  </thead>  <tbody>    <tr>      <th>0</th>      <td>a</td>      <td>b</td>      <td>c</td>      <td>a,b,c</td>    </tr>    <tr>      <th>1</th>      <td>c</td>      <td>NaN</td>      <td>f</td>      <td>c,nan,f</td>    </tr>    <tr>      <th>2</th>      <td>a</td>      <td>b</td>      <td>NaN</td>      <td>a,b,nan</td>    </tr>  </tbody></table>
  
  ## Here is the output I want:
  
  ```Python
  df.concatenate_columns(["cat_1","cat_2","cat_3"],"cat",sep=",",ignore_empty=True)
  ```
  <table border="1" class="dataframe">  <thead>    <tr style="text-align: right;">      <th></th>      <th>cat_1</th>      <th>cat_2</th>      <th>cat_3</th>      <th>cat</th>    </tr>  </thead>  <tbody>    <tr>      <th>0</th>      <td>a</td>      <td>b</td>      <td>c</td>      <td>a,b,c</td>    </tr>    <tr>      <th>1</th>      <td>c</td>      <td>NaN</td>      <td>f</td>      <td>c,f</td>    </tr>    <tr>      <th>2</th>      <td>a</td>      <td>b</td>      <td>NaN</td>      <td>a,b</td>    </tr>  </tbody></table>