Zero services should create a valid [null] SPI

[nickpiggott]

If the epg bridge runs and there are zero services providing SPI information via RadioDNS, the process ends with a warning message but without writing an output file.

This has two implications:

• At the first run, no file is written, and odr-dabmux will not start because the file for the packet data service is missing
• If the SPI has been previously generated, it will never be updated to reflect that fact that no services are providing SPI

Therefore, the process should write a valid SPI file which contains zero services, rather than aborting without writing a file.

[nickpiggott]

The issue is that the multiplex ECC and EID are required, and they're not currently read in from the odr-dabmux configuration file. The code needs to be refactored to read those in from the configuration file and pass them to the generator, rather than relying on copying out the ECC and EID of the first service found.

[nickpiggott]

Fixed - code just committed