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 51.85185% with 13 lines in your changes missing coverage. Please review.

Project coverage is 74.71%. Comparing base (dc446ee) to head (002defa).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...destinationmanager/klaviyobulkupload/apiService.go	42.85%	12 Missing ⚠️
...tionmanager/klaviyobulkupload/klaviyobulkupload.go	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5316      +/-   ##
==========================================
- Coverage   74.80%   74.71%   -0.10%     
==========================================
  Files         437      437              
  Lines       61211    61230      +19     
==========================================
- Hits        45788    45747      -41     
- Misses      12900    12947      +47     
- Partials     2523     2536      +13     

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