Alter function [dbo].[Asd](@hid hierarchyid, @limit int)
    Returns xml
With RTURNS NULL ON NULL INPUT
BEGIN
RETURN
(
   Case when @ limit =0 then null
   ELSE
   (SELECT
       S.hid as "@hid",
       S.level as "@level",
       S.name as "@name",
       (Select
           R. Name  as 'name',
           RTRIM(R.mobile) as 'mobile'
        From [dbo].[V_ACS_a] R
        Where s.hid=r.hid for xml path ('employee'), type) as 'employees',
        Case when @hid=s. Hid. GetAncestor(1) then [dbo].[Asd](s.hid, @limit - 1) end
    From [dbo].[Caompanystructure] s
    Where @hid =s.hid.GetAncestor(1) and Active=1
    For xml path('path'), type
    )
    End
);
End

RTRIM(R.mobile) as 'mobile'

NULLIF(R.mobile, '') as 'mobile'

declare @employees table (name varchar(20), mobile varchar(20))
insert into @employees values
    ('Employee 1', '(123)456-78-90'),
    ('Employee 2', '              '),
    ('Employee 3', '')

select
    e.name as name,
    nullif(e.mobile, '') as mobile
from @employees e
for xml path('employee')

<employee>
  <name>Employee 1</name>
  <mobile>(123)456-78-90</mobile>
</employee>
<employee>
  <name>Employee 2</name>
</employee>
<employee>
  <name>Employee 3</name>
</employee>