`rules:changes` always evaluates to true for external pull request pipelines from github

Summary

rules:changes always evaluates to true for external pull request pipelines from github.

Steps to reproduce

Configure self-hosted gitlab ci/cd for a github repository

Configure external pull request pipeline job to run on changes to a repo subdirectory, e.g.

job:
  rules:
    - if: $CI_PIPELINE_SOURCE == "external_pull_request_event"
      changes:
        - job-dir/**/*

Send a github pull request that doesn't change files in job-dir

Example Project

N/A, self-hosted

What is the current bug behavior?

The job runs.

What is the expected correct behavior?

The job does not run.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

System information
System:         Ubuntu 18.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.7.2p137
Gem Version:    3.1.4
Bundler Version:2.1.4
Rake Version:   13.0.3
Redis Version:  6.0.12
Git Version:    2.31.1
Sidekiq Version:5.2.9
Go Version:     unknown
GitLab information
Version:        13.11.2-ee
Revision:       5898a459003
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     12.5
URL:            https://gitlab.cogtree.com
HTTP Clone URL: https://gitlab.cogtree.com/some-group/some-project.git
SSH Clone URL:  git@gitlab.cogtree.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:
GitLab Shell
Version:        13.17.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
GitLab Shell path:              /opt/gitlab/embedded/service/gitlab-shell
Git:            /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.17.0 ? ... OK (13.17.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes (cluster/worker) ... 1/1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
3/28 ... yes
3/29 ... yes
3/30 ... yes
3/31 ... yes
3/32 ... yes
3/33 ... yes
3/34 ... yes
7/35 ... yes
Redis version >= 5.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.2)
Git version >= 2.31.0 ? ... yes (2.31.1)
Git user has default SSH configuration? ... yes
Active users: ... 2
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Elasticsearch version 7.x (6.4 - 6.x deprecated to be removed in 13.8)? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

I noticed that the satisfied_by? method in the changes clause always returns true for external pull request builds because the pipeline modified_paths method returns nil: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/build/rules/rule/clause/changes.rb#L12.

Is it expected that rules:changes always evaluates to true for external pull request builds?