Add phone codes to countries seed data and fix empty string vs NULL

- Populate phone_code column in countries.csv with ITU-T E.164 codes
- Convert empty strings to NULL for nullable fields in CountriesSeed
- Fix EmptyValuesTestFilterCollection callback to return bool

The phone_code column was empty for all countries, making the
empty-values search demo non-functional. The seed also inserted empty
strings instead of NULL, so IS NULL filters never matched.

Author: dereuromark

config/Seeds/CountriesSeed.php

@@ -23,10 +23,16 @@ public function run(): void {
 		$handle = fopen($file, 'r');
 		$headers = fgetcsv($handle, null, ',', '"', '\\');
 		$rows = [];
+		$nullableFields = ['phone_code', 'zip_regexp', 'address_format', 'timezone'];
 		while (($data = fgetcsv($handle, 1000, ',', '"', '\\')) !== false) {
 			$row = array_combine($headers, $data);
 			unset($row['continent_id']);
 			$row['special'] = '';
+			foreach ($nullableFields as $field) {
+				if (isset($row[$field]) && $row[$field] === '') {
+					$row[$field] = null;
+				}
+			}
 			$rows[] = $row;
 		}
 		fclose($handle);