fix: Add default value for alert_manager_definition

[bryankaraffa]

Description

This fixes and issue with the latest version of this module when no value is provided for alert_manager_definition and the current default value [null] is used:

Error: Missing required argument

  with module.global-zone.module.prometheus.aws_prometheus_alert_manager_definition.this[0],
  on .terraform/modules/global-zone.prometheus/main.tf line 20, in resource "aws_prometheus_alert_manager_definition" "this":
  20:   definition   = var.alert_manager_definition

The argument "definition" is required, but no definition was found.

This error matches the aws provider documentation and definition is required

A empty string value [""] will not work for this either and produces this error:

Error: error creating Prometheus Alert Manager Definition (ws-f6f96b26-b023-41ce-975f-e5c0667f6fc3): ValidationException: Malformed Alertmanager definition.
{
  RespMetadata: {
    StatusCode: 400,
    RequestID: "31676246-1759-465c-94fc-3f954ae6a10d"
  },
  Message_: "Malformed Alertmanager definition."
}

This PR changes the module to use a very basic definition as the default value for alert_manager_definition

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)

[bryantbiggs]

after updating the heredocs format, please run pre-commit run -a to update the documentation

[bryantbiggs]

thanks @bryankaraffa

[antonbabenko]

This PR is included in version 2.1.2 🎉

[github-actions]

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