Fix backup NPE

Fixes #7526

src/org/thoughtcrime/securesms/backup/FullBackupExporter.java

@@ -265,9 +265,12 @@ public class FullBackupExporter extends FullBackupBase {
 
         while ((read = in.read(buffer)) != -1) {
           byte[] ciphertext = cipher.update(buffer, 0, read);
+
+          if (ciphertext != null) {
             outputStream.write(ciphertext);
             mac.update(ciphertext);
           }
+        }
 
         byte[] remainder = cipher.doFinal();
         outputStream.write(remainder);