Super Cache: be more defensive when fetching promo links

[jeherve]

Fixes #34960

Proposed changes:

Avoid trying to fetch specific links when the global is not set.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

I could not replicate the original issue on my end, but submitted this PR based off the info in the original issue, #34960.

It turns out this is not solving their issue, so may not be necessary.

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[donnchawp]

Probably not really needed, but of course, since $wpsc_promo_links is a global variable, there's always a chance it might be modified elsewhere.