Fix T68868: Assert in depsgraph debugging logs

Was happening when tagging for LEGACY_0 was used.

source/blender/depsgraph/DEG_depsgraph.h

@@ -108,6 +108,8 @@ void DEG_graph_on_visible_update(struct Main *bmain, Depsgraph *depsgraph, const
 /* Update all dependency graphs when visible scenes/layers changes. */
 void DEG_on_visible_update(struct Main *bmain, const bool do_time);
 
+/* NOTE: Will return NULL if the flag is not known, allowing to gracefully handle situations
+ * when recalc flag has been removed. */
 const char *DEG_update_tag_as_string(IDRecalcFlag flag);
 
 void DEG_id_tag_update(struct ID *id, int flag);

source/blender/depsgraph/intern/depsgraph_tag.cc

@@ -412,11 +412,15 @@ static void graph_id_tag_update_single_flag(Main *bmain,
 
 string stringify_append_bit(const string &str, IDRecalcFlag tag)
 {
+  const char *tag_name = DEG_update_tag_as_string(tag);
+  if (tag_name == NULL) {
+    return str;
+  }
   string result = str;
   if (!result.empty()) {
     result += ", ";
   }
-  result += DEG_update_tag_as_string(tag);
+  result += tag_name;
   return result;
 }
 
@@ -717,8 +721,7 @@ const char *DEG_update_tag_as_string(IDRecalcFlag flag)
     case ID_RECALC_ALL:
       return "ALL";
   }
-  BLI_assert(!"Unhandled update flag, should never happen!");
-  return "UNKNOWN";
+  return NULL;
 }
 
 /* Data-Based Tagging  */