Handle missing namespace prefix in XML dumps more gracefully

Handle missing namespace prefix in XML dumps more gracefully

If an XML dump of a wiki is exported using dumpBackup.php, and there are
pages in a namespace that is not registered (perhaps because of a missing
extension), they will appear in the dump in the form

<page> ... <title>PageTitle</title> <ns>1234</ns> ... </page>

This caused the ForeignTitle code to raise an undefined offset error,
because it assumed that the <title> element was of the form
"Namespace:PageTitle" when <ns> was nonzero. This assumption is not valid.

Now, the importation of such dumps will no longer throw errors and the
pages will be correctly imported, although possibly to unexpected
locations.

Bug: T114115
Change-Id: I0271435dc208e7ea118339584f8a0e359c96113a