Refactor `getPostcodesArray()`: Remove unreachable defensive checks to resolve coverage gaps

[charlesroper]

Background

In TESTING.md, it is noted that there are untested error handling paths in getPostcodesArray() and suggests the following to improve coverage:

1. Test the preg_split() error path by passing malformed regex patterns
2. Test non-string elements in the postcode array

The Problem

Because the regex pattern /\r\n|\r|\n/ is hardcoded in the function, it can never be "malformed" dynamically by user input. preg_split() will only fail here if PHP suffers a catastrophic error (like running out of memory). Furthermore, preg_split() will only ever return an array of strings in this context, meaning the array elements will never be non-strings.

The checks if ($rows === false) and if (!is_string($row)) are defensive "dead code" that cannot be reached during normal execution, making them virtually impossible to unit test.

The Solution

Instead of trying to devise impossible test scenarios to hit 100% coverage, we should simply remove these unreachable defensive checks. This keeps the codebase leaner, reduces confusion for future maintainers, and automatically resolves the coverage gap.

Proposed Code Change:

Update src/functions.php to remove the dead code:

function getPostcodesArray(?string $postcodes_querystring): array
{
    // If the input is empty, return an empty array.
    if (!$postcodes_querystring) {
        return [];
    }

    // Use a regex to split on the CR+LF or just CR or just LF.
    $rows = preg_split('/\r\n|\r|\n/', $postcodes_querystring);
    $postcodes = [];

    // Normalise the postcode in each row and add non-empty results to the array.
    foreach ($rows as $row) {
        $clean = normalisePostcode($row);
        if ($clean !== '') {
            $postcodes[] = $clean;
        }
    }

    return $postcodes;
}