bug: Exception thrown if `assignees` is not defined

[Hotell]

When some parameters property is not defined it will throw Exception as there is no array to iterate upon:

Example:

Fix:

Various:

• Following line needs to be handled in a defensive way so the program wont throw https://github.com/Naturalclar/issue-action/blob/master/src/setIssueAssignee.ts#L20
• Explicitly document that all fields are required