public EsIndexProperty build() {
            checkArgument(fieldMap != null || type != null, "Both fields and type can not both be null null");
            return new EsIndexProperty(this);
        }
Build and validate instance of {@link EsIndexPropertyBuilder}
@return a new {@link EsIndexPropertyBuilder} with the configuration provided in this builder
common/es/src/main/java/io/apiman/common/es/util/builder/index/EsIndexProperty.java