title: Using `bdate_range` with null values (NaN, NaT)
body: <p>I have three columns with dates, a <code>Begin</code> column and two <code>End</code> columns. I want to use <code>len</code> of <code>bdate_range</code> to find the number of business days between <code>Begin</code> and <code>End</code>. However, using <code>NaT</code> as an argument in <code>bdate_range</code> causes it to throw an error.</p>
  <h3>Setup</h3>
  <pre><code>test = pd.DataFrame({'Begin': ['2014-06-11', '2014-08-05', '2014-09-21', '2014-09-21', '2014-09-21', '-'],
                     'End1': ['2014-06-12', '2014-08-31', 'NaT', '2014-09-30', '2014-09-28', '-'],
                      'End2': ['2014-06-14', '-', '2014-09-30', '-', '2014-09-28', '2014-12-15']})
  
  test['Begin'] = pd.to_datetime(test['Begin'], dayfirst=False, yearfirst=False, errors='coerce', exact=False)
  test['End1'] = pd.to_datetime(test['End1'], dayfirst=False, yearfirst=False, errors='coerce', exact=False)
  test['End2'] = pd.to_datetime(test['End2'], dayfirst=False, yearfirst=False, errors='coerce', exact=False)
  </code></pre>
  <p>Creates this table:</p>
  <pre>
      Begin       End1        End2
  0   2014-06-11  2014-06-12  2014-06-14
  1   2014-08-05  2014-08-31  NaT
  2   2014-09-21  NaT         2014-09-30
  3   2014-09-21  2014-09-30  NaT
  4   2014-09-21  2014-09-28  2014-09-28
  5   NaT         NaT         2014-12-15
  </pre>
  <h3>My Attempt</h3>
  <p>So, in these dataframe I tryed the <code>bdate_range</code> with <code>len</code></p>
  <pre><code>test['bdate_range'] = [len(pd.bdate_range(x,y))for x,y in zip(test['Begin'],test['End1'])]
  .
  .
  .
  ValueError: Neither `start` nor `end` can be NaT
  </code></pre>
  <p><code>fillna</code> not so good...</p>
  <pre><code>test = test.fillna(pd.Timedelta(0))
  test['bdate_range'] = [len(pd.bdate_range(x,y))for x,y in zip(test['Begin'],test['End1'])]
  .
  .
  .
  TypeError: Cannot convert input [0 days 00:00:00] of type &lt;class 'pandas._libs.tslibs.timedeltas.Timedelta'&gt; to Timestamp
  </code></pre>
  <h3>Desired Outcome</h3>
  <p>The ideal outcome is two more columns, in every rows two more fields:</p>
  <ol>
  <li>Begin - End1 differences in workdays, If any value NaT result of the columns will be NaN/NaT</li>
  <li>Begin - End2 differences in workdays, if any value is NaT result of these is NaT/NaN –</li>
  </ol>
  <pre>
      Begin       End1        End2       Begin-End1_bdate Begin-End2_bdate
  0   2014-06-11  2014-06-12  2014-06-14 1                2
  1   2014-08-05  2014-08-31  NaT        18               NaN
  2   2014-09-21  NaT         2014-09-30 NaN              7
  3   2014-09-21  2014-09-30  NaT        7                NaN
  4   2014-09-21  2014-09-28  2014-09-28 5                5
  5   NaT         NaT         2014-12-15 NaN              NaN
  </pre>
  <p>Any, idea?</p>