Change special meaning of pfsDesignId 0x0

When I wrote the original datamodel I included the sentence:

As a convenience to simulators, if the fiberIds in a pfsConfig file are consecutive integers starting at 1 and all ra/dec values are 0.0 the pfsDesignId is taken to be 0x0

In practice we do not use this loophole, as we use pfsConfig files to define the inputs to the simulator. I'd like to remove this dispensation and use 0x0 to mean, "No illuminated fibres". A lawyerly reading of the current rule includes this case, but I think it'd be clearer to change it to:

A pfsDesignId of 0x0 means that no fibres are illuminated.