[fix] Better error handling when MuPDF document has no pages (#3710)

It happens in base so we don't have to worry about it in front.

References https://github.com/koreader/koreader/issues/3705

base

@@ -1 +1 @@
-Subproject commit c629cd88b729edfd13962bd664a0d64b4efc3cb0
+Subproject commit aafc74835bfd1bc53d34d31b8f2efdc9be3d7bbe

frontend/document/pdfdocument.lua

@@ -40,10 +40,6 @@ function PdfDocument:init()
     else
         self:_readMetadata()
     end
-    -- TODO: handle this
-    -- if not (self.info.number_of_pages > 0) then
-        --error("No page found in PDF file")
-    -- end
 end
 
 function PdfDocument:preRenderPage()