Test raising for missing credentials

spec/slack_client_spec.rb

 require "slack_mattermost_emoji/slack_client"
 
 RSpec.describe SlackMattermostEmoji::SlackClient do
+  it 'should raise if credentials are missing' do
+    expect do
+       SlackMattermostEmoji::SlackClient.new(
+        username: '',
+        password: '',
+        domain: '',
+      )
+    end.to raise_error('Slack username, password, and domain are all required')
+  end
+
   it 'should be able to log in to slack using the provided credentials' do
     client = SlackMattermostEmoji::SlackClient.new(
       username: ENV['SLACK_USERNAME'],