[5.1] GuidedTours: No error when joomla-welcome is missing

[Hackwar]

Summary of Changes

When an update of Joomla failed and the joomla-welcome guided tour wasn't added to the database, a big warning is displayed at the top of the backend dashboard. This PR adds another check to make sure we loaded anything in the first place before accessing it.

Testing Instructions

1. Install Joomla
2. Delete the joomla-welcome from #__guidedtours
3. Login to the backend.

Actual result BEFORE applying this Pull Request

Big warning at the top of the screen.

Expected result AFTER applying this Pull Request

No warning.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[obuisard]

I have tested this item ✅ successfully on 8036900

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[Quy]

I have tested this item ✅ successfully on 8036900

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/43139.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/43139.}

[LadySolveig]

Thank you @Hackwar and also for testing @obuisard and @Quy