incorrect rates rebate total when person is not allocated to property

[Br3nda]

When a person is not allocated to a property (they have no role on the Entity named Titled_Property) the calculation returns the maximum possible rebate - even though they are not eligible for that.

e.g. This gives the correct answer

  input:
    persons:
      Fred:
        rates_rebates__combined_income: 32103
        rates_rebates__dependants: 0
    titled_properties:
      whare:
        rates_rebates__rates_total: 2000
        owners: [Fred]

But this gives the maximum ($620)

  input:
    persons:
      Fred:
        rates_rebates__combined_income: 32103
        rates_rebates__dependants: 0
    titled_properties:
      whare:
        rates_rebates__rates_total: 2000

on investigation with @jvdanker , we found this is because Open Fisca adds up the income of all occupants/owners (and there are none) to decide the combined income in that whare is $0 then gives the maximum

Instead, let's require a property has an owner present. If there's none, then either refuse to calculate, or return null, or something else to give an indication that you haven't supplied valid input.