luci-app-statistics: check that uci config sections exist

luci-app-statistics: check that uci config sections exist

A missing or faulty luci_statistics config file may prevent
LuCI access completely already at the login phase as
the statistics config is parsed already then.

The error is like:
  .../statistics/datatree.lua:16: attempt to index upvalue 'sections' (a nil value)

User may be completely locked out of LuCI until he figures out
the need to have a proper luci_statistics config file.

Prevent the error by checking for the existence of
the uci config sections before trying to use values from them.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>