Incorrect calculation of `relative` change

[bartveneman]

real world example:

"atrules.imports.totalUnique": {
            "oldValue": 0,
            "newValue": 0,
            "changed": false,
            "diff": {
                "absolute": 0,
                "relative": 1
            }
        },

the relative: 1 can't be right.

[bartveneman]

Another example:

"selectors.complexity.unique": {
    "diff": [
      {
        "value": {
          "value": 1,
          "count": 1
        },
        "added": false,
        "removed": true,
        "changed": true
      },
      {
        "value": {
          "value": 1,
          "count": 1
        },
        "added": true,
        "removed": false,
        "changed": true
      }
    ],
    "changed": true
  },