Fix issue caused by missing source file in add call

Changes to EvmDatabaseOps for PR ManageIQ/manageiq#17689
will require that the session add method be called on all sessions.  This allows
us to fail gracefully.

lib/gems/pending/util/mount/miq_generic_mount_session.rb

@@ -258,6 +258,10 @@ def add(source, dest_uri)
         logger.info("#{log_header} Skipping add since URI: [#{dest_uri}] already exists")
         return dest_uri
       end
+      unless File.exist?(source)
+        logger.info("#{log_header} Skipping add since file: [#{source}] does not exist")
+        return source
+      end
 
       logger.info("#{log_header} Building relative path: [#{relpath}]...")
       FileUtils.mkdir_p(File.dirname(relpath))