LU-16444 enc: null-enc names cannot be digested form

LU-16444 enc: null-enc names cannot be digested form

When encrypted files have their names encrypted, long names are in
digested form in case access is done without the encryption key. The
digest is base64-encoded, and prepended with '_'.
With null encryption for file names, names are always plain text. In
this case, a legitimate '_' at the start of a name must not be
interpreted as a digested form.

sanity-sec test_54 is improved to test the case of a file whose name
starts with '_'.

Fixes: f18c87cb53 ("LU-13717 sec: handle null algo for filename encryption")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Idaad186afd06cfbabbe1d13e78f083d12876c8ff