fix: handle nullable filter edge cases for empty and not-null values

[isaackaara]

Summary

Fixes two edge cases in FiltersOperator when using dynamic operator filters with ->nullable():

Edge Case 1: Empty filter value

Query: ?filter[my_filter]=
Before: Operator resolved to empty string ""
After: Operator defaults to FilterOperator::EQUAL, producing WHERE column IS NULL

Edge Case 2: Not-null filter with empty value

Query: ?filter[my_filter]=<>
Before: Value was empty string "", producing WHERE column <> ''
After: Value converts to null, producing WHERE column IS NOT NULL

Changes

• Added elseif clause to default to EQUAL operator when value is null in dynamic filter mode
• Added post-processing to convert empty string values to null
• Added two Pest tests covering both scenarios

Credits

Both issues were identified and diagnosed by @pimdongit in #1047 (comment). They provided the proposed fixes -- this PR implements them with test coverage.

Ref: #1047

[freekmurze]

Thanks for the PR and the tests! The edge cases you're fixing are valid.

However, the if ($value === '') { $value = null; } check on line 46-48 is unconditional, meaning it applies to all operator filters, not just dynamic ones. This could change behavior for non-dynamic filters:

For example, AllowedFilter::operator('name', FilterOperator::EQUAL) with ?filter[name]=:

• Before: WHERE name = '' (filter for empty string)
• After: WHERE name IS NULL (different semantics)

The empty-string-to-null conversion should only happen for dynamic filters, where it's the result of stripping the operator prefix (e.g., <> stripped from <> leaves '').

Could you move the check inside the dynamic filter branch instead?

} elseif ($this->filterOperator->isDynamic() && $value !== null) {
    $filterOperator = $this->getDynamicFilterOperator($value);
    $this->removeDynamicFilterOperatorFromValue($value, $filterOperator);
    if ($value === '') {
        $value = null;
    }
} elseif ($this->filterOperator->isDynamic() && $value === null) {
    $filterOperator = FilterOperator::EQUAL;
}

[isaackaara]

Thanks for the clear explanation, @freekmurze. You're right that the unconditional check would change semantics for non-dynamic filters like AllowedFilter::operator('name', FilterOperator::EQUAL) with an empty string value.

Moved the empty-string-to-null conversion inside the isDynamic() && $value !== null branch, so it only fires after stripping the operator prefix leaves an empty string. All existing and new tests pass.

[freekmurze]

Looks good, thanks for the fix and the tests!