Fix notice value might not be defined in alexa #16800

WARNING: plugins/SEO/Metric/Alexa.php(52): Notice - Undefined variable: value - Matomo 4.0.0 - Please report this message in the Matomo forums: https://forum.matomo.org (please do a search first as it might have been reported already) (Module: SEO, Action: getRank, In CLI mode: false)

Was getting above error. Probably because of:

WARNING: Error while getting Alexa SEO stats via fallback method: curl_exec: Resolving timed out after 10069 milliseconds. Hostname requested was: www.alexa.com (Module: SEO, Action: getRank, In CLI mode: false)

fyi @sgiehl will merge this