Fix an issue with 'make ts' when CUSTOMWIZARDS_FILES is empty.

Reviewed-by : Daniel Molkentin
(cherry picked from commit 514b30ec,
this was accidentially committed into 2.0.0 first)

share/qtcreator/translations/extract-customwizards.xq.in

 let $files := ( $$CUSTOMWIZARD_FILES )
 let $prefix := string(\"QT_TRANSLATE_NOOP("ProjectExplorer::CustomWizard", "\")
 let $suffix := concat(\"")\", codepoints-to-string(10))
+where empty($files)
+return
 for $file in $files
     let $doc := doc($file)
     for $text in ($doc/*:wizard/*:description, $doc/*:wizard/*:displayname, $doc/*:wizard/*:displaycategory, $doc/*:wizard/*:fieldpagetitle, $doc/*:wizard/*:fields/*:field/*:fielddescription)