Provide default value for Statistic constructor (oss & pro)

[mathieucarbou]

At the time we create the Stat object, we might not be able to access the stats values yet. So we end up setting an empty string to the value:

    Statistic _appFirmwareStat = Statistic(&_dashboard, "App Firmware", "");
    Statistic _appIdStat = Statistic(&_dashboard, "App Id", "");
    Statistic _appManufacturerStat = Statistic(&_dashboard, "App Manufacturer", "");
    Statistic _appName = Statistic(&_dashboard, "App Name", "");
    Statistic _appVersionStat = Statistic(&_dashboard, "App Version", "");
    Statistic _cpuCoresStat = Statistic(&_dashboard, "CPU Cores", "");
    Statistic _cpuModelStat = Statistic(&_dashboard, "CPU Model", "");

It would be better to support a default constructor with a customisable default, like this:

#ifndef DASH_DEFAULT_STATISTICS_VALUE
  #define DASH_DEFAULT_STATISTICS_VALUE ""
#endif

[...]

Statistic(ESPDash *dashboard, const char *key, const char *value = DASH_DEFAULT_STATISTICS_VALUE);