[GH-208] Add feedback if the list link is empty

[Viktor-Yakovchuk]

Add feedback if the list link is empty Fixes https://github.com/mattermost/mattermost-plugin-autolink/issues/208

Summary

Ticket Link

[mattermost-build]

Hello @Viktor-Yakovchuk,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

Per the Mattermost Contribution Guide, we need to add you to the list of approved contributors for the Mattermost project.

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[Viktor-Yakovchuk]

/check-cla

[Viktor-Yakovchuk]

@hanzei could you please take a look?

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.15 :warning:

Comparison is base (5afadc7) 40.09% compared to head (162bc0a) 39.95%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #211 +/- ## ========================================== - Coverage 40.09% 39.95% -0.15% ========================================== Files 6 6 Lines 833 836 +3 ========================================== Hits 334 334 - Misses 479 482 +3 Partials 20 20 ``` | [Impacted Files](https://codecov.io/gh/mattermost/mattermost-plugin-autolink/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost) | Coverage Δ | | |---|---|---| | [server/autolinkplugin/command.go](https://codecov.io/gh/mattermost/mattermost-plugin-autolink/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost#diff-c2VydmVyL2F1dG9saW5rcGx1Z2luL2NvbW1hbmQuZ28=) | `0.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost)

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[mattermost-build]

This PR has been automatically labelled "stale" because it hasn't had recent activity. A core team member will check in on the status of the PR to help with questions. Thank you for your contribution!

[Viktor-Yakovchuk]

@DHaussermann gentle ping