node-mixin: fix configuration for unset fsSelector/diskDeviceSelector
node-mixin: fix configuration for unset fsSelector/diskDeviceSelector

As per https://github.com/prometheus/node_exporter/pull/1429#discussion_r304210103
we want to fetch all devices and all fs types.

Currently, this is done by setting empty string which breaks most queries which rely on it.

This fixes it by setting the appropriate selector instead of empty string.

Signed-off-by: Sergiusz Urbaniak <7657f3a0c6319088c9e0bc4813152bb1f5637169@gmail.com>

prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter