default to zero width characters in truetype for '.notdef'

when the character is not defined and the corresponding '.notdef' glyph isn't included in a truetype font we now default to a zero width character. it might be that we should use the default/missing width instead but this will work ok for most use-cases.

src/UglyToad.PdfPig/PdfFonts/Simple/TrueTypeStandard14FallbackSimpleFont.cs

@@ -87,7 +87,10 @@
             }
 
             var name = encoding.GetName(characterCode);
-            var metrics = fontMetrics.CharacterMetrics[name];
+            if (!fontMetrics.CharacterMetrics.TryGetValue(name, out var metrics))
+            {
+                return new CharacterBoundingBox(new PdfRectangle(), 0);
+            }
 
             if (overrides?.TryGetWidth(characterCode, out width) != true)
             {