[com_fields] Fix/Improve checks for empty output in field groups

[AndySDH]

Summary of Changes

This PR improves checks for empty output when rendering a field as part of field groups. Some checks were already there but not thorough, and empty output could occur in rare cases.

This makes sure that a field that has an empty value or an empty output does not get displayed in all cases.

One check comes from the original subfields PR (https://github.com/joomla/joomla-cms/pull/22446/files#diff-02a314737553eb71a8516e925b220cce) that was postponed to Joomla 4.
Another extra check I added myself.

Testing Instructions

Not much needed really. Just double checks.

Expected result

Field with empty output should never display

Actual result

Field with empty output could be displayed in rare occurrences

Documentation Changes Required

None

[AndySDH]

Updated with @Quy suggestions, thanks!

[Quy]

I have tested this item ✅ successfully on ff61169

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[richard67]

I have tested this item ✅ successfully on ff61169

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/25145.}

[Quy]

RTC

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[viocassel]

I have tested this item ✅ successfully on d6c16be

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/25145.}

[AndySDH]

To merge in 3.9.8?

Edit: 3.9.9 now I guess :D

[Quy]

@AndySDH Please apply the suggested changes. It will not require retesting.

[AndySDH]

I wasn't too sure the suggested changes made much sense as there's no need to compare for data type when we are comparing to an empty string.

But ok, I applied them.

[AndySDH]

Reminder about this for 3.9.9

[ghost]

Reminder about this for 3.9.9

calling Release Lead @HLeithner to be sure.

[HLeithner]

thx

[ghost]

@AndySDH can you please mail me [email protected]

[AndySDH]

@AndySDH can you please mail me [email protected]

I did