fixed crash when <font> tag with unavailable font name is present

It is possible that HTML contains tag `<font name="...">` and the font with the given name is not available (`fontWithName:size:` returns `nil`). In such a case we will try to set `NSFontAttributeName` with nil `value`, which results in a crash. It can be simply fixed by using `systemFontOfSize:` when `fontWithName:size:` returns `nil`.