Description of problem:

ipa delegation-mod is not failing when --attrs= is used with no value.  Instead it simply removes the Attribute entry from the delegation altogether.  I would have expected this to error out and not allow entering an empty attrs to delete it from the delegation.

# ipa delegation-find testdelegation
--------------------
1 delegation matched
--------------------
  Delegation name: testdelegation
  Permissions: write
  Attributes: mobile
  Member user group: dnegroup
  User group: ipausers
----------------------------
Number of entries returned 1
----------------------------

# ipa delegation-mod testdelegation --attrs=
------------------------------------
Modified delegation "testdelegation"
------------------------------------
  Delegation name: testdelegation
  Permissions: write
  Member user group: dnegroup
  User group: ipausers


Version-Release number of selected component (if applicable):

ipa-server-2.2.0-101.20120118T0229zgitbb854ce.el6.x86_64
ipa-admintools-2.2.0-101.20120118T0229zgitbb854ce.el6.x86_64
389-ds-base-libs-1.2.9.14-1.el6.x86_64
389-ds-base-1.2.9.14-1.el6.x86_64

How reproducible:

always

Steps to Reproduce:
1.  <setup ipa server>
2.  kinit admin
3.  ipa delegation-add testdelegation --membergroup=admins --group=ipausers --attrs=mobile
4.  ipa delegation-mod testdelegation --attrs=

  
Actual results:

Removes Attributes from Delegation.

------------------------------------
Modified delegation "testdelegation"
------------------------------------
  Delegation name: testdelegation
  Permissions: write
  Member user group: dnegroup
  User group: ipausers


Expected results:

Disallow this behavior and return a descriptive error message.

Additional info:

The same occurs for:

ipa delegation-mod testdelegation --attrs=""

and for:

ipa delegation-mod testdelegation --attrs=" "