FIX Prevent array to string conversion for default_sort on a fixed field

[robbieaverill]

This shows a regression in 3.7.x-dev where you can no longer define a default_sort that is a "fixed field" (e.g. Created from DataObject) without throwing an "array to string conversion" error.

One option is to cast empty arrays in Object::parse_class_spec to an empty string so this doesn't happen. I'm not sure where the regression happened, but this test shows it.

[kinglozzer]

I suspect this is related to #7945. It looks like DataObject::db() will always return an array for fixed fields 😕. Perhaps this could be refactored to something like

// Outside the parent foreach()
$fields = DataObject::database_fields(get_class($this));

// ...

$columnSpec = $fields[$column];
list($fieldType) = SS_Object::parse_class_spec($columnSpec);

[tractorcow]

This is a little obscure, where the underlying reason is "sometimes code passes in an empty array in some edge case". It'd be better to never call this method with an invalid arg, rather than silently ignoring the error.

[tractorcow]

I've fixed this with #8146

The issue is nothing to do with parse_class_spec, it's a distraction.

database_fields() doesn't return anything for ID so you're out of luck there anyway.

The fix was to use hasOwnTableDatabaseField() instead. See #8146

[robbieaverill]

Thank you!