make it cheaper when there is no path in ContentPath (elastic#135645)

It makes a bit cheaper to call ContentPath#pathAsText when there is actually no path as we are not copying the string ]into the string buffer.

Author: iverase

server/src/main/java/org/elasticsearch/index/mapper/ContentPath.java

@@ -58,6 +58,9 @@ public boolean isWithinLeafObject() {
     }
 
     public String pathAsText(String name) {
+        if (index == 0) {
+            return name;
+        }
         sb.setLength(0);
         for (int i = 0; i < index; i++) {
             sb.append(path[i]).append(DELIMITER);