Fix PHP 8 fatal/warning on first opt-in without paid plans

[Tobias-Conrad]

What this fixes

On a fresh first opt-in of a product/store, the SDK can raise two PHP 8 diagnostics before the account carries any plan list (see #879, reproduced on SDK 2.13.1/2.13.4 and current develop):

1. Fatal error — _store_plans() calls count( $this->_plans ) while $this->_plans is still its default false:

   Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, bool given
   in .../includes/class-freemius.php
   

2. Warning — FS_Site::__construct() reads $site->plan_id, which the install payload can omit:

   Warning: Undefined property: stdClass::$plan_id in .../includes/entities/class-fs-site.php on line 123
   

Root cause

$this->_plans defaults to false (declaration private $_plans = false;). _set_account() only assigns it when a plan list is passed:

if ( false !== $plans ) {
    $this->_plans = $plans;
}

On a first opt-in without paid plans, $plans stays false, and _store_plans() is still reached via _store_account().

The change (minimal, two spots)

• _store_plans(): encrypt an empty list when _plans is not an array, instead of calling count() on false.
• FS_Site::__construct(): read plan_id via isset() and default to null, matching how the property is handled elsewhere.

Both use the exact convention already present in the class (! is_array( $this->_plans ), see _get_plan_by_id() / get_plan_by_name()), so this is consistent with the existing code style rather than a new pattern.

Verification

• php -l clean on both changed files.
• Change is a no-op for the normal (array) case — behaviour is unchanged whenever _plans is a real array.
• Reproduced the fatal on a clean WordPress + PHP 8 first opt-in before the fix; the warning/fatal no longer occur after it.

No other code paths were touched: is_plan() / is_trial_plan() etc. are only reached once is_registered() is true, at which point _plans is already an array — so they are not on the reported crash path.

Fixes #879.