Detect blank squares in text nodejs

So am using the Arial Unicode Ms font in nodejs. Sometimes a few user's nicknames appear as blank squares on top of the image I draw the text on. So what I want is with some way to eliminate those squares.

i) Can I detect somehow if my font has that character?

ii) Since I am using the GraphicsMagick module, is there some way to replace with empty string those characters?

iii) Can I with combine fonts, to fill in those characters?

Generally I want to find a way to remove those blank squares or actually show the characters, any of the above would do.

via nonerth