fix imagesrcset bug

[kara]

28% of Angular 17 apps using the image directive have been missing performance data due to errors. Looking into it, it seems that link.imagesrcset is undefined in some of these cases. Grabbing the attribute instead of the prop with link.getAttribute fixes the issue.

See example failure: https://www.webpagetest.org/result/240618_AiDc8Y_E9D/3/details/ See example fix: https://www.webpagetest.org/result/240618_AiDcZ3_E78/2/details/

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Test websites:

• https://exploreti.com/

[github-actions[bot]]

Custom metrics for https://almanac.httparchive.org/en/2022/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240618_G6_6 Changed custom metrics values: ```json {} ```

Custom metrics for https://exploreti.com/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240618_DQ_7 Changed custom metrics values: ```json {} ```