handle null charstring names

it appears charstring definitions in adobe type 1 fonts can omit the charstring name. in this case we set the name to the string value of the charstring index.

src/UglyToad.PdfPig.Fonts/Type1/CharStrings/Type1CharstringDecryptedBytes.cs

@@ -25,7 +25,7 @@
         {
             Bytes = bytes ?? throw new ArgumentNullException(nameof(bytes));
             Index = index;
-            Name = name;
+            Name = name ?? index.ToString();
             Source = SourceType.Charstring;
         }