Fix typos in JSON MRR output

mysql-test/r/explain_json.result

@@ -1492,7 +1492,7 @@ EXPLAIN
       "buffer_type": "flat",
       "buffer_size": "256Kb",
       "join_type": "BKA",
-      "mrr_type": "; Rowid-ordered scan"
+      "mrr_type": "Rowid-ordered scan"
     }
   }
 }
@@ -1535,7 +1535,7 @@ ANALYZE
       "buffer_type": "flat",
       "buffer_size": "256Kb",
       "join_type": "BKA",
-      "mrr_type": "; Rowid-ordered scan",
+      "mrr_type": "Rowid-ordered scan",
       "r_filtered": 100
     }
   }

sql/sql_explain.cc

@@ -1809,7 +1809,10 @@ void Explain_table_access::append_tag_name(String *str, enum explain_extra_tag t
       str->append(STRING_WITH_LEN(" join"));
       str->append(STRING_WITH_LEN(")"));
       if (bka_type.mrr_type.length())
+      {
+        str->append(STRING_WITH_LEN("; "));
         str->append(bka_type.mrr_type);
+      }
 
       break;
     }

sql/sql_join_cache.cc

@@ -2620,7 +2620,8 @@ static void add_mrr_explain_info(String *str, uint mrr_mode, handler *file)
                                            sizeof(mrr_str_buf));
   if (len > 0)
   {
-    str->append(STRING_WITH_LEN("; "));
+    if (str->length())
+      str->append(STRING_WITH_LEN("; "));
     str->append(mrr_str_buf, len);
   }
 }