[Bug] fix bug with empty filter values returning no results in system/service

[fearful-symmetry]

What does this PR do?

This fixes a bug where if the system/service module is running on an OS with an older version of systemd, and a user hasn't configured pattern_filter, the metricset will return no results. When using the systemd ListUnitsFiltered API call, we don't bypass the pattern_filter in cases where the filter is empty.

Why is it important?

It's a bug, the metricset won't return any data.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Build and Run on CentOS 7, which uses a sufficiently old version of systemd.
• Enable the system/service metricset
• make sure service.pattern_filter isn't configured.
• Make sure we get data.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #19812 updated]

• Start Time: 2020-07-14T14:21:20.950+0000

• Duration: 82 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3089
Skipped	705
Total	3794

Steps errors

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• Name: Install Go 1.14.4

  • Description: .ci/scripts/install-go.sh

  • Duration: 2 min 22 sec

  • Start Time: 2020-07-14T14:45:47.039+0000

  • log

• Name: Report to Codecov

  • Description: curl -sSLo codecov https://codecov.io/bash for i in auditbeat filebeat heartbeat libbeat metricbeat packetbeat winlogbeat journalbeat do FILE="${i}/build/coverage/full.cov" if [ -f "${FILE}" ]; then bash codecov -f "${FILE}" fi done

  • Duration: 2 min 22 sec

  • Start Time: 2020-07-14T14:54:20.272+0000

  • log

• Name: Install Go 1.14.4

  • Description: .ci/scripts/install-go.sh

  • Duration: 2 min 22 sec

  • Start Time: 2020-07-14T14:45:46.142+0000

  • log

• Name: Report to Codecov

  • Description: curl -sSLo codecov https://codecov.io/bash for i in auditbeat filebeat heartbeat libbeat metricbeat packetbeat winlogbeat journalbeat do FILE="${i}/build/coverage/full.cov" if [ -f "${FILE}" ]; then bash codecov -f "${FILE}" fi done

  • Duration: 2 min 22 sec

  • Start Time: 2020-07-14T15:39:09.927+0000

  • log