Not getting any result while using if condition and getting the result if not using the if condition

alter procedure ListEmployeedtls_Paging
@FirstName varchar(200)

as
begin
Declare @result nvarchar(4000);
Declare @orderby nvarchar(700);
Declare @PageSize nvarchar(200);
Declare @PageNum nvarchar(200);
Declare @Offset nvarchar(200);




set @orderby=' order by e.EmpId desc';

set @PageSize=10
set @PageNum=1
set @Offset=@PageSize*(@PageNum-1)



set @Result='select Top '+@PageSize+' e.EmpId
,(e.FirstName+'' ''+e.LastName) as EmpName
,e.EmpMobile
,e.EmpGender
,e.EmpAddress
,e.IsActive
,e.EmpEmail
,e.EmpSalary
,d.DeptName

,convert(varchar(10),e.JoiningDate,103) as JoiningDate
, convert(varchar(10),e.EmpDateOfBirth,103) as EmpDateOfBirth

,e.SupervisorId
,(s.FirstName+'+' '+'+s.LastName) as SupervisorName
,g.GradeName
,r.Rolename 
,e.Password


from Employeedtls e left outer join Department d on d.DeptId=e.DepartmentId
left outer join Grade g on g.GradeId=e.GradeId 
left outer join User_Role r on r.RoleId=e.RoleId
left outer  join Employeedtls s on e.SupervisorId=s.EmpId 
where 1=1  
and e.EmpId not in

 (select  top '+@Offset+' e1.EmpId
from Employeedtls e1 
left outer join Department d1 on d1.DeptId=e1.DepartmentId
left outer  join Employeedtls s1 on e1.SupervisorId=s1.EmpId
left outer join Grade g1 on g1.GradeId=e1.GradeId 
left outer join User_Role r1 on r1.RoleId = e1.RoleId where 1=1) '


if((@FirstName is not null) or @FirstName!='''')
begin
set @Result=@Result+' and  e.FirstName like ''% @FirstName %'''
end

	

print @Result



	
	
print @Result+ @orderby

exec (@Result + @orderby)
end