Mobile: use map extent center if map center has not been set

StatefulMap tries to use a map attribute that is not set by default in the mobile scaffold:
https://github.com/camptocamp/c2cgeoportal/blob/1.4/c2cgeoportal/scaffolds/create/%2Bpackage%2B/static/mobile/config.js_tmpl#L53-L58
therefore triggering an "undefined" JS error.