bug: slice types should be omitted when empty, else they are 'null' in resulting JSON

[devilleweppenaar]

The resulting JSON for the checkly.Check struct contains null values for slice values that are not provided. Examples of this are:

• Check.Locations
• Check.EnvironmentVariables
• Check.Tags
• Request.Headers
• Request.QueryParameters
• Request.Assertions

We should add the ,omitempty annotation so that these values are not passed as null in the JSON.