fix(provider/aws): No-op ami tag update when no tags provided

The AWS `createTags` API requires at least one tag to be provided.

Rather than error out, let's just skip the update entirely and return
success.

clouddriver-aws/src/main/groovy/com/netflix/spinnaker/clouddriver/aws/deploy/ops/UpsertAmiTagsAtomicOperation.groovy

@@ -86,6 +86,12 @@ class UpsertAmiTagsAtomicOperation implements AtomicOperation<Void> {
         .withResources(amiId)
         .withTags(tags)
 
+      if (!tags) {
+        // createTags expects at least one tag to have been provided
+        task.updateStatus BASE_PHASE, "Skipping empty tags update for ${amiId} in ${region}"
+        return true
+      }
+
       task.updateStatus BASE_PHASE, "Updating tags for ${amiId} in ${region}..."
       amazonEC2.createTags(createTagsRequest)
       task.updateStatus BASE_PHASE, "Tags updated for ${amiId} in ${region}..."

clouddriver-aws/src/test/groovy/com/netflix/spinnaker/clouddriver/aws/deploy/ops/UpsertAmiTagsAtomicOperationSpec.groovy

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2017 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.netflix.spinnaker.clouddriver.aws.deploy.ops
+
+import com.amazonaws.services.ec2.AmazonEC2
+import com.amazonaws.services.ec2.model.CreateTagsRequest
+import com.amazonaws.services.ec2.model.Tag
+import com.netflix.spinnaker.clouddriver.aws.deploy.description.UpsertAmiTagsDescription
+import com.netflix.spinnaker.clouddriver.aws.security.AmazonClientProvider
+import com.netflix.spinnaker.clouddriver.data.task.Task
+import com.netflix.spinnaker.clouddriver.data.task.TaskRepository
+import spock.lang.Specification;
+
+class UpsertAmiTagsAtomicOperationSpec extends Specification {
+  def task = Mock(Task)
+  def amazonEC2 = Mock(AmazonEC2)
+  def amazonClientProvider = Mock(AmazonClientProvider)
+
+  def setup() {
+    TaskRepository.threadLocalTask.set(task)
+  }
+
+  def "should skip update if no tags provided"() {
+    given:
+    def description = new UpsertAmiTagsDescription()
+    def operation = new UpsertAmiTagsAtomicOperation(description)
+    operation.amazonClientProvider = amazonClientProvider
+
+    when:
+    operation.upsertAmiTags("us-west-1", "my-ami-id", [])
+
+    then:
+    1 * task.updateStatus("UPSERT_AMI_TAGS", "Skipping empty tags update for my-ami-id in us-west-1")
+    1 * amazonClientProvider.getAmazonEC2(null, "us-west-1") >> { return amazonEC2 }
+    0 * amazonEC2.createTags(_)
+    0 * _
+
+    when:
+    operation.upsertAmiTags("us-west-1", "my-ami-id", [new Tag("my-key", "my-value")])
+
+    then:
+    1 * task.updateStatus("UPSERT_AMI_TAGS", "Updating tags for my-ami-id in us-west-1...")
+    1 * task.updateStatus("UPSERT_AMI_TAGS", "Tags updated for my-ami-id in us-west-1...")
+    1 * amazonClientProvider.getAmazonEC2(null, "us-west-1") >> { return amazonEC2 }
+    1 * amazonEC2.createTags({ CreateTagsRequest createTagsRequest ->
+      createTagsRequest.tags == [new Tag("my-key", "my-value")]
+      createTagsRequest.resources == ["my-ami-id"]
+    })
+    0 * _
+  }
+}