Ah, the blank result just means that your model doesn't report 'health' values. You should see something like the following (on most models) when checking 'resource' values:
[admin@router-01] > /system resource print oid
used-memory: .1.3.6.1.2.1.25.2.3.1.6.65536
uptime: .1.3.6.1.2.1.1.3.0
build-time: .1.3.6.1.4.1.14988.1.1.7.6.0
total-memory: .1.3.6.1.2.1.25.2.3.1.5.65536
cpu-frequency: .1.3.6.1.4.1.14988.1.1.3.14.0