Fix love.font.newBMFontRasterizer when it's given a single file param.

src/modules/font/wrap_Font.cpp

@@ -154,7 +154,7 @@ int w_newBMFontRasterizer(lua_State *L)
 			lua_pop(L, 1);
 		}
 	}
-	else
+	else if (!lua_isnoneornil(L, 2))
 	{
 		convimagedata(L, 2);
 		image::ImageData *id = luax_checktype<image::ImageData>(L, 2);