Fix the NaN error when the minuteStepping is produce empty <td> tags.

If the `minuteStepping` is as such 60 minutes is not divisible by 4 without remainder (there is four columns in the table) then there are some empty cell (`<td>`) in the last row.
For example `minuteStepping` is 10 then the minute selector table is the following:

00 | 10 | 20 | 30
---|----|----|---
40 | 50 | | 

If you click on the empty cells then you get an `NaN` as the selected date.

![bdtpicker-nan](https://cloud.githubusercontent.com/assets/591103/4606195/bc71a838-5213-11e4-9c3c-f5c256d1c247.png)