AlertmanagerConfig with missing options causes Alertmanager to crash

Always

A cluster administrator has enabled monitoring for user-defined projects.
CMO 

~~~
 config.yaml: |
    enableUserWorkload: true
    prometheusK8s:
      retention: 7d
~~~

A cluster administrator has enabled alert routing for user-defined projects. 

UWM cm / CMO cm 

~~~
apiVersion: v1
kind: ConfigMap
metadata:
  name: user-workload-monitoring-config
  namespace: openshift-user-workload-monitoring
data:
  config.yaml: |
    alertmanager:
      enabled: true 
      enableAlertmanagerConfig: true
~~~

verify existing config: 

~~~
$ oc exec -n openshift-user-workload-monitoring alertmanager-user-workload-0 -- amtool config show --alertmanager.url http://localhost:9093  
global:
  resolve_timeout: 5m
  http_config:
    follow_redirects: true
  smtp_hello: localhost
  smtp_require_tls: true
  pagerduty_url: https://events.pagerduty.com/v2/enqueue
  opsgenie_api_url: https://api.opsgenie.com/
  wechat_api_url: https://qyapi.weixin.qq.com/cgi-bin/
  victorops_api_url: https://alert.victorops.com/integrations/generic/20131114/alert/
  telegram_api_url: https://api.telegram.org
route:
  receiver: Default
  group_by:
  - namespace
  continue: false
receivers:
- name: Default
templates: []
~~~

create alertmanager config without options "smtp_from:" and "smtp_smarthost"

~~~
apiVersion: monitoring.coreos.com/v1alpha1
kind: AlertmanagerConfig
metadata:
  name: example
  namespace: example-namespace
spec:
  receivers:
    - emailConfigs:
        - to: some.username@example.com
      name: custom-rules1
  route:
    matchers:
      - name: alertname
    receiver: custom-rules1
    repeatInterval: 1m
~~~

check logs for alertmanager: the following error is seen. 

~~~
ts=2023-09-05T12:07:33.449Z caller=coordinator.go:118 level=error component=configuration msg="Loading configuration file failed" file=/etc/alertmanager/config_out/alertmanager.env.yaml err="no global SMTP smarthost set"
~~~ 

Alertmamnager fails to restart.

CRD should be pre validated.

Reproducible with and without user workload Alertmanager.