Uninitialized values output 0 rather than “NaN” or something more appropriate

[scruss]

On startup, energy and temperature fields are not initialized, as the packets are sent asynchronously. For example, three initial records might look like:

[ 87369] Energy:     0 Wh, Power: 576 W, Temp:  0 F
[119189] Energy:     0 Wh, Power: 604 W, Temp: 47 F
[151017] Energy: 17792 Wh, Power: 607 W, Temp: 47 F

In the first line, neither temperature nor energy is initialized. In the second, energy alone isn't.

These should really be something non-numeric, but easy to parse. NaN may not be ideal.