Description extractor raise error under certain document structures

[7sDream]

#import "@preview/shiroa:0.3.1": text-support
#import text-support: plain-text

#plain-text(table(), limit: 255)

$ typst compile .\test.typ                                                                                                                 
error: type none has no method `clusters`                                                                                                  
   ┌─ @preview/shiroa:0.3.1\supports-text-internal.typ:72:15
   │
72 │     assert(res.clusters().len() <= limit, message: "plain-text length limit exceeded")
   │                ^^^^^^^^

help: error occurred in this call of function `plain-text`
  ┌─ \\?\test.typ:4:1
  │
4 │ #plain-text(table(), limit: 255)
  │  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is because, for code here, empty table has a empty children array, make parts be an empty array, then join into none.

This is also true when the table is not empty, but all it's cells will be converted into empty string (only contains image, for example).