Can't provide customRole.actions[*].resources[*].collection as empty string

[Freyert]

Describe the bug
Because the SDK uses value types instead of pointer types for type Resource.(Db|Collection) users of the SDK will not be able to send empty strings for fields such as collection or db as they will be fully omitted when serialized to JSON.

I've put together an example in "The Go Playground" to demonstrate the issue: https://play.golang.org/p/OHNEv_7HIcc

{"actions":[{"action":"DROP_INDEX","resources":

[{"db":"admin"}]}], //<--- should have "collection": ""

"inheritedRoles":null,"roleName":"500Error"}

Expected behavior
If I set Resource.Collection = "" I should have "collection": "" in the JSON output.

{"actions":[{"action":"DROP_INDEX","resources":

[{"db":"admin", "collection": "" }]}], // "collection": "" exists

"inheritedRoles":null,"roleName":"500Error"}

Logs
https://play.golang.org/p/OHNEv_7HIcc

Desktop (please complete the following information):

• OS: [e.g. macOS]
• Lib Version [e.g. 0.6.0]

Additional context
This should be an issue everywhere the SDK is being used. For example, I think it will be an issue here in the terraform provider:

https://github.com/mongodb/terraform-provider-mongodbatlas/blob/master/mongodbatlas/resource_mongodbatlas_custom_db_role.go#L118

[Freyert]

Working with pointer values and structs can be fairly frustrating because you can't directly set a &"" in a struct without first assigning "" to a variable.

The aws-sdk-go gets around this by providing a function that takes a string and returns a pointer (aws.String)

I've wondered why they do that for the longest time and now I know :D