pi/netsender: fix JSON error in first time auto config

Issue #81 resolved
Trek Hopton created an issue

If a Pi’s netsender.conf has a registered MAC and device key, the rest of the configuration should be pulled automatically from the device on vidgrind. This doesn’t happen, instead it fails and on the Pi we see the error message:

{"level":"warn","time":"2024-01-19T11:43:25.914+1030","caller":"rv/main.go:205","message":"rv: Run Failed. Retrying...","error":"JSON decoding error"}