dbaas: Protect against setting empty tags.

[andrewsomething]

There seems to have been a change in the DBaaS API. It is now returning:

  "tags": [
   ""
  ]

in the response body for MongoDB clusters without any tags applied. This is wrong, but we should still protect against this here. Without doing so, we end up with a diff and a failing acceptance test:

$ make testacc TESTARGS='-run=TestAccDigitalOceanDatabaseCluster_MongoDBPassword'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanDatabaseCluster_MongoDBPassword -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanDatabaseCluster_MongoDBPassword
=== PAUSE TestAccDigitalOceanDatabaseCluster_MongoDBPassword
=== CONT  TestAccDigitalOceanDatabaseCluster_MongoDBPassword
    resource_digitalocean_database_cluster_test.go:409: Step 1/1 error: After applying this test step, the plan was not empty.
        stdout:
        
        
        Terraform used the selected providers to generate the following execution
        plan. Resource actions are indicated with the following symbols:
          ~ update in-place
        
        Terraform will perform the following actions:
        
          # digitalocean_database_cluster.foobar will be updated in-place
          ~ resource "digitalocean_database_cluster" "foobar" {
                id           = "1f706e9a-ada4-434c-9058-b7086a2a79e4"
                name         = "tf-acc-test-xda03f78tl"
              ~ tags         = [
                  - "",
                ]
                # (14 unchanged attributes hidden)
            }
        
        Plan: 0 to add, 1 to change, 0 to destroy.
    testing_new.go:63: Error running post-test destroy, there may be dangling resources: DatabaseCluster still exists
--- FAIL: TestAccDigitalOceanDatabaseCluster_MongoDBPassword (385.09s)

These changes applied resolve that problem.

[bentranter]

👍

[scotchneat]

nice catch