server : allow text-only slot save/restore with mtmd (#25076)

tools/server/server-common.h

@@ -207,6 +207,9 @@ public:
 
     bool empty() const { return tokens.empty(); }
 
+    // true if the sequence actually contains image/audio chunks.
+    bool has_media() const { return !map_idx_to_media.empty(); }
+
     void clear() {
         map_idx_to_media.clear();
         tokens.clear();