fix(cli): accept nullable function import map paths

[7ttp]

TL;DR

fixes nullable function import map paths

prob

• While testing the ported functions deploy flow here: feat(cli): port functions deploy #5561
  I found that functions list could fall back to request_failed because the management api can return
  import_map_path: null

Go already accepted that null shape, but the generated ts contract only accepted a string

basically this updates the generated function response contracts to accept null for import_map_path
and adds a integration coverage around it!

ref

• smol regression of: (feat: add supabase functions list #5185)

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