6720240: IOB exception when getting font metrics of hershey font

Reviewed-by: bae, prr

src/share/classes/sun/font/NullFontScaler.java

@@ -76,7 +76,7 @@ class NullFontScaler extends FontScaler {
     }
 
     int getNumGlyphs() throws FontScalerException {
-        return 0;
+        return 1;
     }
 
     int getMissingGlyphCode() throws FontScalerException {