branch resource_rules fail when a date condition is specified

We are trying to set up gitlab-triage system on our local gitlab instance (we are using community edition v15.8) and, while doing so, ran into an error when trying to set up a triage policy for branch resources.

From our dry-run tests, we found that specifying date condition for branch resources, results in an error from branch_date_filter.rb file.

Error message

gitlab-triage --dry-run --token $GITLAB_API_TOKEN --host-url $CI_SERVER_URL --source groups --source-id $TRIAGE_SOURCE_PATH
/usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/filters/branch_date_filter.rb:49:in `resource_value': undefined method `[]' for nil:NilClass (NoMethodError)
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/filters/branch_date_filter.rb:61:in `calculate'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:340:in `block in filter_resources'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:333:in `select'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:333:in `filter_resources'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:302:in `block in resources_for_rule'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/expand_condition.rb:13:in `each'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/expand_condition.rb:13:in `perform'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:279:in `resources_for_rule'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:183:in `block in process_rules'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:182:in `each'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:182:in `process_rules'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:79:in `block in perform'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:70:in `each'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/lib/gitlab/triage/engine.rb:70:in `perform'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/bin/gitlab-triage:18:in `block in <top (required)>'
	from /usr/local/lib/ruby/2.7.0/set.rb:328:in `each_key'
	from /usr/local/lib/ruby/2.7.0/set.rb:328:in `each'
	from /usr/local/bundle/gems/gitlab-triage-1.28.0/bin/gitlab-triage:11:in `<top (required)>'
	from /usr/local/bundle/bin/gitlab-triage:23:in `load'
	from /usr/local/bundle/bin/gitlab-triage:23:in `<main>'
*********************************************
Executing policies from .triage-policies.yml.
*********************************************
Performing a dry run.
=========================================================================================================
Triaging the `hahn-schickard/software-sollutions/application-engineering/internal/opc_ua_dev_group` group
=========================================================================================================
-----------------------------------------
Processing summaries & rules for branches
-----------------------------------------
-----------------------------------------------------
Gathering resources for rule: **Stale branch policy**
-----------------------------------------------------
.
* Found 1 resources...
* Filtering resources...
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

We use the default CI-CD pipeline configuration file from the readme:

.gitlab-ci.yml

image: ruby:2.7

stages:
  - triage

dry-run:triage:
  stage: triage
  script:
    - gem install gitlab-triage
    - gitlab-triage --help
    - gitlab-triage --dry-run --token $GITLAB_API_TOKEN --host-url $CI_SERVER_URL --source groups --source-id $TRIAGE_SOURCE_PATH
  when: manual
  except:
    - schedules

run:triage:
  stage: triage
  script:
    - gem install gitlab-triage
    - gitlab-triage --token $GITLAB_API_TOKEN --host-url $CI_SERVER_URL --source groups --source-id $TRIAGE_SOURCE_PATH
  only:
    - schedules

The triage policy that generates this error:

.triage-policies.yml

resource_rules:
  branches:
    rules:
      - name: Stale branch policy
        conditions:
          date:
            attribute: committed_date
            condition: older_than
            interval_type: months
            interval: 6
          protected: false
        actions:
          issue:
            title: |
              Review stale branch {{ title }}
            description: |
              {{ author }}, the branch {{ full_reference }} had no new commits for 6 months. 

              Please consider merging/closing this branch.

Removing the date condition from the .triage-policies.yml file, results in a successful pipeline