fix: handle missing sign parameter in EPay notify callback

[onesyue]

Summary

• PHP 8 throws ErrorException: Undefined array key "sign" when accessing $params['sign'] on an empty/incomplete POST body
• When the payment callback POST body is lost through CDN/proxy layers (e.g., Cloudflare forwarding, reverse proxy misconfiguration), $request->input() returns an empty array
• This causes a 500 error response, and the payment gateway interprets it as a failure and retries indefinitely
• Fix: use null coalescing operator (??) and return false early to gracefully reject invalid callbacks

Changes

plugins/Epay/Plugin.php line 82:

// Before
$sign = $params['sign'];

// After
$sign = $params['sign'] ?? null;
if (!$sign) return false;

Test plan

• Verify normal EPay payment callback still works (sign present and valid)
• Verify callback with empty POST body returns false instead of 500
• Verify callback with missing sign key returns false instead of throwing exception

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