In case rocket_atf_warmup_links filter is given a wrong input, the fetch mechanism should default to 10 URLs + home

[hanna-meda]

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Describe the bug
Using any other input but integer inside rocket_atf_warmup_links filter, filter will not work (which is expected) but nor will there be any URLs fetched.

To Reproduce
Steps to reproduce the behavior:

1. Add snippet:
   add_filter( 'rocket_atf_warmup_links_number', function () { return ; });
2. For return value use any non-integer inputs (i.e null, 0, -10, 3.14, [ ], "")
3. Save and activate filter
4. Clear Critical Images
5. Check wp-rocket-debug.log.html file
6. Notice there are no "Add to queue request arguments" logs inside it, so no links were fetched.

Expected behavior
In the case where rocket_atf_warmup_links_number filter is given the wrong kind of input, then the fetch mechanism should default to 10 URLs + home.

Screenshots
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Additional context
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Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming