WPSC: Check that there is cached content to display in browser

[donnchawp]

If a website is using an NFS/Samba share to host their cached content, there may be a race condition when serving cache pages. This can happen when one server is serving a page, it checks for the existence of the cache file, and before the file can be read, another server deletes the file.

The check is repeated multiple times because different processing is done each time, sometimes with and without gzipping the data.

Fixes https://wordpress.org/support/topic/edge-case-can-lead-to-blank-page-being-served/

Proposed changes:

• Make sure that $cachefiledata isn't false before serving a cached page.

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?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

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

Testing instructions:

• This would be very difficult to test as it would require two servers, one serving a page, and the other modifying it.
• However, file_get_contents returns false if there's an error, and it can't read the file, so this should be easy to visually check.

[github-actions]

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