Use String.blank in jdk.javadoc where applicable

There are couple of places where manual cycle is used to check if String contains non-whitespaces characters.
Since Java 11 we have String.isBlank() which could be used instead.

    boolean isAllWhiteSpace(String body) {
        for (int i = 0 ; i < body.length(); i++) {
            if (!Character.isWhitespace(body.charAt(i)))
                return false;
        }
        return true;
    }

    boolean hasNonWhitespace(TextTree tree) {
        String s = tree.getBody();
        for (int i = 0; i < s.length(); i++) {
            Character c = s.charAt(i);
            if (!Character.isWhitespace(s.charAt(i)))
                return true;
        }
        return false;
    }