fix(Employee): set user image and validate user id only if user data is found

[ruchamahabal]

Setting user image in employee and validating user details fails if a stale/non-existent user id is linked to employee master

Traceback (most recent call last):
  File "apps/frappe/frappe/desk/form/save.py", line 21, in savedocs
    doc.save()
  File "apps/frappe/frappe/model/document.py", line 287, in save
    return self._save(*args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 309, in _save
    return self.insert()
  File "apps/frappe/frappe/model/document.py", line 240, in insert
    self.run_before_save_methods()
  File "apps/frappe/frappe/model/document.py", line 971, in run_before_save_methods
    self.run_method("validate")
  File "apps/frappe/frappe/model/document.py", line 868, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1161, in composer
    return composed(self, method, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1144, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "apps/frappe/frappe/model/document.py", line 862, in 
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "apps/erpnext/erpnext/hr/doctype/employee/employee.py", line 57, in validate
    self.validate_user_details()
  File "apps/erpnext/erpnext/hr/doctype/employee/employee.py", line 73, in validate_user_details
    if data.get("user_image") and self.image == '':
AttributeError: 'NoneType' object has no attribute 'get'

Already fixed for develop in #29024
Not adding tests; Its a defensive fix