I tested on XCode 10.2.
Effectively, it is not nil but it is empty.
Here is the content.
label.attributedText Optional(Label{
NSColor = "UIExtendedSRGBColorSpace 0 0 0 1";
NSFont = "<UICTFont: 0x7fa3efc0d3a0> font-family: \".SFUIText\"; font-weight: normal; font-style: normal; font-size: 17.00pt";
NSParagraphStyle = "Alignment 4, LineSpacing 0, ParagraphSpacing 0, ParagraphSpacingBefore 0, HeadIndent 0, TailIndent 0, FirstLineHeadIndent 0, LineHeight 0/0, LineHeightMultiple 0, LineBreakMode 4, Tabs (\n 28L,\n 56L,\n 84L,\n 112L,\n 140L,\n 168L,\n 196L,\n 224L,\n 252L,\n 280L,\n 308L,\n 336L\n), DefaultTabInterval 0, Blocks (\n), Lists (\n), BaseWritingDirection -1, HyphenationFactor 0, TighteningForTruncation NO, HeaderLevel 0";
NSShadow = "NSShadow {0, -1} color = {(null)}";
})
If some attributed text was defined, I get
label.attributedText Optional(someText{
})
Probably, you should file a bug report against documentation