luci-mod-status: fix NaN errors in realtime graphs

When I use Realtime Graphs I see following NaN errors:

    Error: <polyline> attribute points: Expected number, "0,298 0,NaN 5,NaN 10,NaN…".
    Error: <polyline> attribute points: Expected number, "0,298 0,NaN 5,NaN 10,NaN…".
    wireless.js?v=git-21.105.40538-2da37c2:19 Error: <polyline> attribute points: Expected number, "0,298 0,NaN 5,NaN 10,NaN…".
    eval @ wireless.js?v=git-21.105.40538-2da37c2:19
    Promise.then (async)
    eval @ wireless.js?v=git-21.105.40538-2da37c2:8
    step @ luci.js?v=git-21.105.40538-2da37c2:91
    start @ luci.js?v=git-21.105.40538-2da37c2:87
    add @ luci.js?v=git-21.105.40538-2da37c2:82
    pollData @ wireless.js?v=git-21.105.40538-2da37c2:7
    render @ wireless.js?v=git-21.105.40538-2da37c2:24
    Promise.then (async)
    __init__ @ luci.js?v=git-21.105.40538-2da37c2:148
    super @ luci.js?v=git-21.105.40538-2da37c2:22
    Anonymous42Class @ luci.js?v=git-21.105.40538-2da37c2:12
    (anonymous) @ luci.js?v=git-21.105.40538-2da37c2:181
    Promise.then (async)
    compileClass @ luci.js?v=git-21.105.40538-2da37c2:177
    Promise.then (async)
    require @ luci.js?v=git-21.105.40538-2da37c2:183
    instantiateView @ ui.js?v=git-21.105.40538-2da37c2:311
    (anonymous) @ wireless:47
    Promise.then (async)
    (anonymous) @ wireless:46

Signed-off-by: Lukasz Baj <l.baj@celerway.com>
[adjust subject]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>