formatBreadcrumb() accepts undefined as string

[cbratschi]

Describe the Bug

The following code casts any field value to a string:

label = lastDoc[useAsTitle] as string

https://github.com/payloadcms/payload/blob/main/packages/plugin-nested-docs/src/utilities/formatBreadcrumb.ts#L23

The code should convert all values to valid strings:

label = lastDoc[useAsTitle] ?? '' as string

Link to the code that reproduces this issue

https://github.com/cbratschi/payload

Reproduction Steps

See above.

Which area(s) are affected? (Select all that apply)

plugin: nested-docs

Environment Info

Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  payload: 3.13.0
Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:05:23 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6031
  Available memory (MB): 65536
  Available CPU cores: 16