Fix TrueTypeFontUnicode when metrics contains null

TrueTypeFontUnicode WriteFont() method throws when metrics contains
null values. This commit fixes the issue by filtering out null values

src/iTextSharp.LGPLv2.Core/iTextSharp/text/pdf/TrueTypeFontUnicode.cs

@@ -324,7 +324,7 @@ internal override void WriteFont(PdfWriter writer, PdfIndirectReference piref, o
             tmp.Add(o);
         }
 
-        var metrics = tmp.ToArray();
+        var metrics = tmp.Where(m => m != null).ToArray();
         Array.Sort(metrics, this);
         PdfIndirectReference indFont = null;
         PdfObject pobj = null;