"ensure => absent" leads to evaluation error if "source_format => 'sources'"

[dhs-rec]

Describe the Bug

With

Apt::Source { source_format => 'sources' }

set globally, a simple

apt::source { 'somerepo':
  ensure => 'absent',
}

leads to the following error:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Resource Statement, Apt::Setting[sources-somerepo]: parameter 'content' expects a value of type Undef or String[1], got String (file: /etc/puppet/code/environments/production/modules/apt/manifests/source.pp, line: 358) on node host.example.org

Expected Behavior

No evaluation error

Steps to Reproduce

Steps to reproduce the behavior:
Run the example above

Environment

• Version 10.0.1
• Platform Debian

Additional Context

The problematic line(s) seems to be

...
        'absent': {
          $header = undef
          $source_content = undef
        }
...
    content       => "${header}${source_content}",

which lead to content being set to an empty(?) string.