[5.1] Fix PHP warnings on newly created custom fields

[Quy]

Pull Request for Issue #40681 .

Summary of Changes

Separate conditions of the if statement to apply to new fields vs existing fields accordingly.

When creating a new field and the Search Index setting is Don't make searchable, the last 2 conditions are not applicable thus php warnings.

PHP Warning:  Attempt to read property "params" on null in \administrator\components\com_fields\src\Model\FieldModel.php on line 123
PHP Warning:  Trying to access array offset on value of type null in \administrator\components\com_fields\src\Model\FieldModel.php on line 123

Testing Instructions

Install Blog Sample Data.
No errors in PHP log.

Add a new custom field.
Save field.
No errors in PHP log.

[laoneo]

Three times nested if conditions do look for me as not the right fix. Probably adding some comments to the existing one would be a better solution.

[Quy]

@Hackwar Please review as it originated with #38650. Thanks.

[exlemor]

I have tested this item ✅ successfully on 51b02c0

I was able to test this successfully - tested every field type - no error log entries.

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[cybersalt]

Try as I may, I could not get these errors even before the patch.

[viocassel]

I have tested this item ✅ successfully on 51b02c0

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[Quy]

RTC

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[bembelimen]

Thx