Omit reviewers from environment payload when empty

An explicit empty `reviewers: []` was sent to the API, clearing any
existing required reviewers on an already-provisioned environment. Treat
an empty array like an absent key and omit it from the payload.

Author: phelma

lib/rake_github/tasks/environments/provision.rb

@@ -45,7 +45,7 @@ def environment_options(client, org, environment)
         end
 
         def resolve_reviewers(client, org, reviewers)
-          return nil if reviewers.nil?
+          return nil if reviewers.nil? || reviewers.empty?
 
           reviewers.map { |reviewer| resolve_reviewer(client, org, reviewer) }
         end

spec/rake_github/tasks/environments/provision_spec.rb

@@ -267,6 +267,24 @@ def define_task(opts = {}, &block)
             .with(repository, 'release', {}))
   end
 
+  it 'omits reviewers when an empty array is supplied' do
+    repository = 'org/repo'
+
+    client = stub_github_client
+
+    define_task(
+      repository:,
+      access_token: 'some-token',
+      environments: [{ name: 'release', reviewers: [] }]
+    )
+
+    Rake::Task['environments:provision'].invoke
+
+    expect(client)
+      .to(have_received(:create_or_update_environment)
+            .with(repository, 'release', {}))
+  end
+
   it 'retains falsy optional values such as wait_timer zero' do
     repository = 'org/repo'