Fix failing creating new Alert in Control > Explorer

[hstastna]

Fixes BZ:
https://bugzilla.redhat.com/show_bug.cgi?id=1654896

Related PR:
#4947

What:
This PR fixes failing creating a new Alert in Control > Explorer page.

Steps to reproduce:

1. Go to Control > Explorer > Alerts accordion > All Alerts
2. Select Configuration > Add new alert
3. Fill in some info (Description and Driving Event are required),
   check the Show on Timeline checkbox, in Timeline Event section instead of Send an E-mail, for example
4. Click on Add button
   => error:

[----] I, [2018-12-06T17:22:47.963847 #3514:2abc52c6f44c]  INFO -- : Started POST "/miq_policy/alert_edit?button=add" for ::1 at 2018-12-06 17:22:47 +0100
[----] I, [2018-12-06T17:22:48.011375 #3514:2abc52c6f44c]  INFO -- : Processing by MiqPolicyController#alert_edit as JS
[----] I, [2018-12-06T17:22:48.011707 #3514:2abc52c6f44c]  INFO -- :   Parameters: {"button"=>"add"}
[----] F, [2018-12-06T17:22:48.032725 #3514:2abc52c6f44c] FATAL -- : Error caught: [NoMethodError] undefined method `[]' for nil:NilClass
/home/hstastna/manageiq/manageiq-ui-classic/app/controllers/miq_policy_controller/alerts.rb:650:in `alert_valid_record?'
/home/hstastna/manageiq/manageiq-ui-classic/app/controllers/miq_policy_controller/alerts.rb:26:in `alert_edit_save_add'
/home/hstastna/manageiq/manageiq-ui-classic/app/controllers/miq_policy_controller/alerts.rb:60:in `alert_edit'

Notes:
Failing depends on which info is (not) filled in while creating a new alert - it does not happen always.
Mostly failing does not occur if all the info is filled in properly, including the emails.

---

Before: (not able to create a new Alert, nothing happens in the UI)

After:(creating a new Alert successful)

[hstastna]

@miq-bot add_label bug, blocker, hammer/yes

[hstastna]

@miq-bot remove_label wip

[hstastna]

@skateman Could you, please, check the spec? 😏 Thanks!

[miq-bot]

Checked commits hstastna/manageiq-ui-classic@d9906ea~...37f4cf7 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 0 offenses detected
Everything looks fine. 🍪

[skateman]

@hstastna looks good

[simaishi]

Hammer backport details:

$ git log -1
commit 8fb6d104870ed9088bc4553c506595c433462387
Author: Milan Zázrivec <mzazrivec@redhat.com>
Date:   Mon Dec 10 12:04:53 2018 +0100

    Merge pull request #5056 from hstastna/Alert_creation_fail_nil
    
    Fix failing creating new Alert in Control > Explorer
    
    (cherry picked from commit 8615b7476e823dd7ad9d5c7369006f2b472a58f8)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1654896