fix: json marshal l errors when parsing poll response from klaviyo

[koladilip]

Description

There is a bug in the Klaviyo implementation where, at times, we receive an empty import ID from the upload. However, when we poll with an empty import ID, we encounter parsing errors because Klaviyo returns a list response when the import ID is empty.

Linear Ticket

https://linear.app/rudderstack/issue/INT-2820/klaviyo-bulk-upload-at-times-they-are-running-into-errors
Resolves INT-2820

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 73.53%. Comparing base (f908e21) to head (85c1961).

Files with missing lines	Patch %	Lines
...destinationmanager/klaviyobulkupload/apiService.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5315      +/-   ##
==========================================
- Coverage   73.57%   73.53%   -0.04%     
==========================================
  Files         424      424              
  Lines       59935    59943       +8     
==========================================
- Hits        44095    44077      -18     
- Misses      13370    13402      +32     
+ Partials     2470     2464       -6     

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