Check for imageinfo rather than missing or invalid

Because shared-repo images are reported as missing, do not check
for that status. And to make things simpler, use the same handling
for invalid ones as well, and just check to see if any imageinfo
metadata has been returned (that's all we're interested in, after
all).

This is a follow-up to GH #512.

Bug: T372956

src/BookProvider.php

@@ -304,10 +304,9 @@ public function getCover( $cover ): ?Picture {
 			'iiurlwidth' => $width,
 			'formatversion' => 2,
 		] )->wait();
-		// Give up for invalid cover titles or those that do not exist.
+		// Give up if no imageinfo is returned (e.g. invalid or missing title).
 		if ( !isset( $response['query']['pages'] )
-			|| isset( $response['query']['pages'][0]['missing'] )
-			|| isset( $response['query']['pages'][0]['invalid'] )
+			|| !isset( $response['query']['pages'][0]['imageinfo'][0] )
 		) {
 			return null;
 		}