Error when invitation for a new user got accepted

Introduced by !39628 (merged)

• Happens to new user (email/user account not known)
• Invitation still works but users will receive a 500
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/invites_controller.rb#L107

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1786087/?referrer=gitlab_plugin

PatternMatchingError: Contract violation for argument 5 of 7:
        Expected: (Num or nil),
        Actual: "00685ab4331d354eac35837d616becba"
        Value guarded in: SnowplowTracker::Tracker::track_struct_event
        With Contract: String, String, Maybe, Maybe, Maybe, Maybe, SnowplowTracker::Timestamp => SnowplowTracker::Tracker
        At: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/snowplow-tracker-0.6.1/lib/snowplow-tracker/tracker.rb:252 
  contracts.rb:48:in `block in <class:Contract>'
    fail data[:contracts].failure_exception.new(failure_msg(data), data)
  contracts.rb:151:in `failure_callback'
    return DEFAULT_FAILURE_CALLBACK.call(data)
  contracts/call_with.rb:19:in `block in call_with'
    return unless Contract.failure_callback(:arg => arg,
  contracts/call_with.rb:13:in `times'
    (@args_contract_index || args.size).times do |i|
  contracts/call_with.rb:13:in `call_with'
    (@args_contract_index || args.size).times do |i|
...
(144 additional frame(s) were not displayed)

PatternMatchingError: Contract violation for argument 5 of 7:
        Expected: (Num or nil),
        Actual: "00685ab4331d354eac35837d616becba"
        Value guarded in: SnowplowTracker::Tracker::track_struct_event
        With Contract: String, String, Maybe, Maybe, Maybe, Maybe, SnowplowTracker::Timestamp => SnowplowTracker::Tracker
        At: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/snowplow-tracker-0.6.1/lib/snowplow-tracker/tracker.rb:252 
  contracts.rb:48:in `block in <class:Contract>'
    fail data[:contracts].failure_exception.new(failure_msg(data), data)
  contracts.rb:154:in `failure_callback'
    fetch_failure_callback.call(data)
  contracts/method_handler.rb:143:in `rescue in block in redefine_method'
    ::Contract.failure_callback(error.data, false)
  contracts/method_handler.rb:136:in `block in redefine_method'
    begin
  gitlab/tracking.rb:32:in `event'
    snowplow.track_struct_event(category, action, label, property, value, context, (Time.now.to_f * 1000).to_i)
...
(142 additional frame(s) were not displayed)

ContractError: Contract violation for argument 5 of 7:
        Expected: (Num or nil),
        Actual: "00685ab4331d354eac35837d616becba"
        Value guarded in: SnowplowTracker::Tracker::track_struct_event
        With Contract: String, String, Maybe, Maybe, Maybe, Maybe, SnowplowTracker::Timestamp => SnowplowTracker::Tracker
        At: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/snowplow-tracker-0.6.1/lib/snowplow-tracker/tracker.rb:252 
  contracts/method_handler.rb:145:in `rescue in rescue in block in redefine_method'
    raise final_error.to_contract_error
  contracts/method_handler.rb:142:in `rescue in block in redefine_method'
    begin
  contracts/method_handler.rb:136:in `block in redefine_method'
    begin
  gitlab/tracking.rb:32:in `event'
    snowplow.track_struct_event(category, action, label, property, value, context, (Time.now.to_f * 1000).to_i)
  invites_controller.rb:107:in `track_experiment'
    Gitlab::Tracking.event(
...
(141 additional frame(s) were not displayed)

ContractError: Contract violation for argument 5 of 7:
        Expected: (Num or nil),
        Actual: "00685ab4331d354eac35837d616becba"
        Value guarded in: SnowplowTracker::Tracker::track_struct_event
        With Contract: String, String, Maybe, Maybe, Maybe, Maybe, SnowplowTracker::Timestamp => SnowplowTracker::Tracker
        At: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/snowplow-tracker-0.6.1/lib/snowplow-tracker/tracker.rb:252