[0.14.21] Energy usage has to be positive when it is 0

[matjojo]

As the title says, when the energy usage is set to "0kW" the error message says it has to be positive.
Really Really minor, but still.

steps to reproduce:

add a new entity that uses energy(like assembling machine)
set node energy_usage to "0kW"
launch game
see error

[Rseding91]

Not a bug: https://simple.wikipedia.org/wiki/Positive_number a positive number is one that is > 0.