Empty mapping objects are added when a previous mapping is cleared

[oanat]

Steps to reproduce:

• create a group or user mapping, e.g.

• delete the values for "last_name" and "cn:
• click on "Save"

Result:

• the mapping section displays 3 empty objects instead of 2

• when doing multiple updates on the same mapping the user can end up with many empty objects

Expected result: no extra empty objects should be added in this case.

[ilieandriuta]

I've reproduced this issue on when just fill a mapping and just saving it afterwards (tested on Active Directory 1.7, Group Mapping).

[acotiuga]

Fixed in 581778f for #26.