Fix defining Mask for grayscale transparent PNG files

Author: blikblum

lib/image/png.js

@@ -49,10 +49,10 @@ class PNGImage {
 
     // For PNG color types 0, 2 and 3, the transparency data is stored in
     // a dedicated PNG chunk.
-    if (this.image.transparency.grayscale) {
+    if (this.image.transparency.grayscale != null) {
       // Use Color Key Masking (spec section 4.8.5)
       // An array with N elements, where N is two times the number of color components.
-      const val = this.image.transparency.greyscale;
+      const val = this.image.transparency.grayscale;
       return this.obj.data['Mask'] = [val, val];
 
     } else if (this.image.transparency.rgb) {