ParserOutput::collectMetadata: Properly handle non-scalar page properties

ParserOutput::setPageProperty() was deprecated for use with non-string
values in 1.42 but there are still callers out there; handle these cases
without the implicit cast to string which ::setUnsortablePageProperty()
would do via its argument type hint.

Bug: T374046
Bug: T373920
Followup-To: I68c28b0d5d23decc058a46c55e767a83c80452f8
Followup-To: I9a235ae828c2cadc9d2c619760f759e51ba73874
Change-Id: I52ecce78fcee8b18cf9d7ea848946f29e2d8b51b