TODO-4858: part #2: Do not print garbage in Optimizer Trace, best_access_path

Patch from Monty:
When we have "cause": "no predicate for first keypart", then
it doesn't make sense to print "rows" or  "cost" as these values are
not defined. (We printed whatever was the left-over from the previous
considered access method).

mysql-test/main/opt_trace.result

@@ -10826,8 +10826,6 @@ S
 {
     "access_type": "ref",
     "index": "PRIMARY",
-    "rows": 1.79769e308,
-    "cost": 1.79769e308,
     "chosen": false,
     "cause": "no predicate for first keypart"
 }

sql/sql_select.cc

@@ -8164,7 +8164,6 @@ best_access_path(JOIN      *join,
   SplM_plan_info *spl_plan= 0;
   table_map spl_pd_boundary= 0;
   Range_rowid_filter_cost_info *filter= 0;
-  const char* cause= NULL;
   enum join_type best_type= JT_UNKNOWN, type= JT_UNKNOWN;
 
   disable_jbuf= disable_jbuf || idx == join->const_tables;  
@@ -8193,6 +8192,7 @@ best_access_path(JOIN      *join,
     KEYUSE *start_key=0;
     TABLE *table= s->table;
     double best_records= DBL_MAX;
+    const char* cause= NULL;
     uint max_key_part=0;
 
     /* Test how we can use keys */
@@ -8754,7 +8754,8 @@ best_access_path(JOIN      *join,
                                table->key_info[filter->key_no].name);
         }
       }
-      trace_access_idx.add("rows", records).add("cost", tmp);
+      if (!cause)
+        trace_access_idx.add("rows", records).add("cost", tmp);
 
       if (tmp + 0.0001 < best_time - records/TIME_FOR_COMPARE)
       {
@@ -8773,7 +8774,6 @@ best_access_path(JOIN      *join,
         trace_access_idx.add("chosen", false)
                         .add("cause", cause ? cause : "cost");
       }
-      cause= nullptr;
     } /* for each key */
     records= best_records;
   }