Fix OOB coming while Urdu text rendering

DEVSIX-6314

Author: vitali-pr

io/src/main/java/com/itextpdf/io/font/otf/Glyph.java

@@ -304,7 +304,7 @@ public String getUnicodeString() {
      * @return the Unicode char sequence that corresponds to this glyph
      */
     public char[] getUnicodeChars() {
-        if (chars != null) {
+        if (chars != null && chars.length > 0) {
             return chars;
         } else {
             return REPLACEMENT_CHARACTERS;