Try Unicode Character ‘ZERO WIDTH SPACE’ (U+200B). It is not a Whitespace according to WP: Whitespace#Unicode
The code of StringUtils.isBlank will not bother it:
public static boolean isBlank(String str) {
int strLen;
if (str == null || (strLen = str.length()) == 0) {
return true;
}
for (int i = 0; i < strLen; i++) {
if ((Character.isWhitespace(str.charAt(i)) == false)) {
return false;
}
}
return true;
}