PoItem::formatSingular doesn't add linebreak on untranslated strings

Problem/Motivation

When using PoItem to export translation strings, the linebreak output differs between translated and untranslated strings.

Steps to reproduce

$untranslated = new PoItem();
$untranslated->setSource("");

// msgid ""
// msgstr ""
$untranslated->__toString():

$translated = new PoItem();
$translated->setSource("");
$translated->setTranslation("");

// msgid ""
// msgstr ""
//
$translated->__toString();

Proposed resolution

Append line break to singular untranslated string output for consistency with other cases.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet