Don't require that it_interval be valid if it_value is set to zero - as per documentation.
Don't require that it_interval be valid if
it_value is set to zero - as per documentation.

Suggested by: ache & bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase