[5.3] php 8.4 compatibility

[brianteeman]

Summary of Changes

PHP Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated

Testing Instructions

Code review or use the cypress integration tests for mfa with php 8.4
npx cypress run --spec '.\tests\System\integration\administrator\components\com_users\Mfa.cy.js'

Actual result BEFORE applying this Pull Request

php deprecated message in the logs

Expected result AFTER applying this Pull Request

no deprecated message in the logs

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[alikon]

I have tested this item ✅ successfully on 714f3d6

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[joomdonation]

I have tested this item ✅ successfully on 714f3d6

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45169.}

[joomdonation]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45169.}

[laoneo]

Thanks!