What does this implement/fix?

When test compiling FTL on a machine without swap, we've seen the following result in the tests:

GET /api/info/system (BODY auth):
  - FTL's reply (<class 'NoneType'>) does not match defined type (number) in system.memory.swap.%used

This is caused by a division-by-zero error which lead to a null being returned in the JSON. There are two obvious fixes:

1. Change the API specs to accept returning a null here, or
2. Ensure that in case of no swap, 0 is returned instead (0% swap used).

The latter seems superior as it needs less logic on the client side to handle this.

Even when we will never see a system with no RAM, the logic is applied here as well - just as a precaution because division-by-zero is never a good thing.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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