fix(waiver): handle empty date_of_birth strings in individual fee insert

MySQL rejects empty strings for DATE columns. The approve form sends
empty strings for persons without a DOB (spouses, temporary members).
Convert empty strings to null for date_of_birth, relationship, status,
and notes fields before INSERT.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Modules/Waiver/Services/WaiverProcessor.php

@@ -491,6 +491,8 @@ final class WaiverProcessor
                 $feeAmount = number_format((float) $feeRate, 2, '.', '');
             }
 
+            $dob = trim((string) ($fee['date_of_birth'] ?? ''));
+
             $db->insert('waiver_individual_fees', [
                 'waiver_request_id' => $waiverId,
                 'person_type' => $fee['person_type'],
@@ -502,10 +504,10 @@ final class WaiverProcessor
                 'fee_amount' => $feeAmount,
                 'age_years' => $fee['age_years'] ?? null,
                 'age_category' => $fee['age_category_code'] ?? $fee['age_category'] ?? null,
-                'date_of_birth' => $fee['date_of_birth'] ?? null,
-                'relationship' => $fee['relationship'] ?? null,
-                'status' => $fee['status'] ?? null,
-                'notes' => $fee['notes'] ?? null,
+                'date_of_birth' => $dob !== '' ? $dob : null,
+                'relationship' => ($fee['relationship'] ?? '') !== '' ? $fee['relationship'] : null,
+                'status' => ($fee['status'] ?? '') !== '' ? $fee['status'] : null,
+                'notes' => ($fee['notes'] ?? '') !== '' ? $fee['notes'] : null,
             ]);
 
             if ($fee['person_type'] === 'spouse') $spouseTotal = bcadd($spouseTotal, $feeAmount, 2);