`escape_rdn` crashes on empty input

[sersorrel]

It feels like this should not happen:

>>> ldap3.utils.dn.escape_rdn("")
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/.../venv/lib/python3.10/site-packages/ldap3/utils/dn.py", line 399, in escape_rdn
    if rdn[0] == '#' or rdn[0] == ' ':
IndexError: string index out of range

(given e.g. escape_filter_chars("") just produces an empty string)

[strazto]

escape_rdn isn't even documented 😱

I'm assuming it mitigates injection vulns?