Fix issue with optional ids param

[dmiller9911]

• Guard against nullish ids param.

• add strict null checks to prevent this in the future.

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Bug fix

• What is the current behavior? (You can also link to an open issue here) https://github.com/jongold/figma-js/issues/40

  
          params: Object.assign(Object.assign({}, params), { ids: params.ids.join(',') }),
                                                                             ^

TypeError: Cannot read property 'join' of undefined

* **What is the new behavior (if this is a feature change)?**
Allows params and params.ids to be optional again

* **Other information**:

[chrisdrackett]

thank you!