Don't report an empty media selection as a successful pick

Both PhotoLibraryPicker and DocumentPicker reported failures via .error
but still emitted .selectedMediaAtURLs([]) when every selected item
failed to load or export (e.g. an iCloud photo that isn't downloaded, or
a document that fails the security-scoped temp copy) - an empty "success"
that consumers had to defend against. Skip that emission in both, and
report .error for PhotoLibraryPicker's previously silent
missing-content-type case so the user gets feedback.

ElementX/Sources/Screens/MediaPickerScreen/DocumentPicker.swift

@@ -90,6 +90,11 @@ struct DocumentPicker: UIViewControllerRepresentable {
                 }
             }
             
+            guard !selectedURLs.isEmpty else {
+                // Every picked document failed to copy; each failure was already surfaced via .error.
+                return
+            }
+            
             documentPicker.callback(.selectedMediaAtURLs(selectedURLs))
         }
     }