fix: optional option no longer trigger an error when omitted

The `coverageReporter.instrumenter` object can be passed to the karma
configuration to override the automatic instrumenters inferring (since this
[PR]). However, when the `coverageReporter` key was omitted, a type error was
emitted.

[PR]: https://github.com/karma-runner/karma-coverage/pull/79
Merge pull request #81 from karma-runner/fix-omitted-option-error

fix: optional option no longer trigger an error when omitted