Fix deprecated warning in contenthistory

[chmst]

Pull Request for Issue # .

Summary of Changes

Fix deprecated warning in contenthistory. It is caused by the null value for example in checked_out_time.

Testing Instructions

See screenshot. Oben the contenthistory of an article and open one if the versions.

Actual result BEFORE applying this Pull Request

See screenshot

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

[brianteeman]

I have tested this item ✅ successfully on f66757c

issue replicated and confirmed resolved with this pr

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[viocassel]

I have tested this item ✅ successfully on f66757c

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[alikon]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/40213.}

[obuisard]

Thank you Christiane @chmst for the PR!