fix(r/burn_alert): handle existing remote description with default description value

[jharley]

When upgrading to 0.28.0, if you have an existing burn alert that had a description set (via the UI, most likely) but the description was yet to be added to HCL config you will get the following error:

Error: Provider produced inconsistent result after apply
        
        When applying changes to honeycombio_burn_alert.test, provider
        "provider[\"registry.terraform.io/hashicorp/honeycombio\"]" produced an
        unexpected new value: .description: was cty.StringVal(""), but now
        cty.StringVal("test description").

Fix is to ensure we send the empty string when we marshal our JSON

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.55%. Comparing base (c8084fc) to head (c69c49c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #575   +/-   ##
=======================================
  Coverage   75.55%   75.55%           
=======================================
  Files          87       87           
  Lines        7286     7286           
=======================================
  Hits         5505     5505           
  Misses       1429     1429           
  Partials      352      352           

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