[4.0] Fix js error when previewing admin template

[infograf768]

Pull Request for Issue #29120

Summary of Changes

Use conditional for the $content variable

Testing Instructions

Make sure that the Preview Module Positions is set to enabled
Click on the eye to open the preview if in template style or Preview Toolbar in templates manager

Before patch

js error. See #29120

After patch

No more error

[brianteeman]

I have tested this item ✅ successfully on 673f18a

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[richard67]

I have tested this item ✅ successfully on 673f18a

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/29124.}

[richard67]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/29124.}

[wilsonge]

Thanks!