e2fsck: Don't print the PO header when printing problems with PROMPT_NULL

e2fsck: Don't print the PO header when printing problems with PROMPT_NULL

A few e2fsck problem messages supply their own prompt, and set a
prompt value of PROMPT_NULL.  We have to check for this case, and not
pass the null string to _(), since that will result in the translation
header getting printed, like this:

Run journal anywayProject-Id-Version: e2fsprogs
Report-Msgid-Bugs-To: FULL NAME <EMAIL@ADDRESS>
POT-Creation-Date: 2008-02-28 21:45-0500
PO-Revision-Date: 2006-05-23 11:12+0000
Last-Translator: Somebody32 <som32@mail.ru>
Language-Team: Russian <ru@li.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n%10>=2 &&
n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;
X-Launchpad-Export-Date: 2008-05-28 00:43+0000
X-Generator: Launchpad (build Unknown)
<y>? yes

Addresses-Launchpad-Bug: #246892

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>