DXCDT-540: Prevent null string literal when importing custom text prompts

[willvedd]

🔧 Changes

When using Terraform config-driven imports with the auth0_prompt_custom_text resource, there can be cases where the body property gets generated literally as "null". This occurred because the Management API returns an empty object string literal "{}" for unconfigured custom text prompts and would get interpreted somewhere in the Terraform SDK as nil which would cause a bit of havoc when marshalling back to a string.

With this PR, the generated custom text prompt looks like this:

resource "auth0_prompt_custom_text" "fr_signup" {
  body     = "{}"
  language = "fr"
  prompt   = "signup"
}

This is preferred because it matches the output of the Management API.

🔬 Testing

Added unit tests to enable empty object as body property. Manually verified that config-driven import works as expected.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

Merging #821 (061dc5c) into v1 (d292300) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##               v1     #821   +/-   ##
=======================================
  Coverage   90.38%   90.38%           
=======================================
  Files          99       99           
  Lines       13525    13528    +3     
=======================================
+ Hits        12224    12227    +3     
  Misses        924      924           
  Partials      377      377           

Files Changed	Coverage Δ
internal/auth0/prompt/flatten.go	67.85% <100.00%> (+3.85%)	⬆️