Update glyph name calculation for code 0 in Type1 font program

Author: MaximPlusov

src/main/java/org/verapdf/pd/font/type1/Type1FontProgram.java

@@ -317,7 +317,7 @@ private void initializeEncoding() {
 
     @Override
     public String getGlyphName(int code) {
-        if (code > 0 && code < encoding.length) {
+        if (code >= 0 && code < encoding.length) {
             return encoding[code];
         } else {
             return null;