RDV: Fix the performance issue on AT sites

[BogdanUngureanu]

Fixes Automattic/wp-calypso#98603

When the ExPlat API returns a null assignment value we don't cache the result, thus making an API request on every page load.

Since only a12s should be un-assigned to a variation, this PR adds "treatment" variation as a fallback value.

Proposed changes:

• Default to treatment for users that don't have an assignment returned

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

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

Testing instructions:

• Install the change on your Atomic site
• Make sure you get treatment
• Make sure that we don't make an API request on each page load (using the debug tool)

[github-actions]

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[github-actions]

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[rcrdortiz]

When the experiment_name's value is NULL we don't go inside this branch.

[rcrdortiz]

We can improve/simplify the checks by:

if ( isset( $data['variations'][ $experiment_name ] ) ) {
    $variation = $data['variations'][ $experiment_name ];
    update_user_option( get_current_user_id(), $option_name, $variation, true );
    $is_enabled = 'treatment' === $variation;
} elseif ( isset( $data['variations'] ) ) {
  /**
   * If the variations array is set but the variation value is null chances are this is an a11n (since ExPlat returns null for a12s).
   * 
   * We want to set the treatment for all a12s.
   */
    update_user_option( get_current_user_id(), $option_name, 'treatment', true );
    $is_enabled = true;
} else {
    $is_enabled = false;
}

[BogdanUngureanu]

Yeah, that could work too.

[rcrdortiz]

Besides the nit I left, everything works as expected now.