Fix customer import: normalize blank email to null, not empty string

The UNIQUE index on (CompanyId, Email) uses HasFilter([Email] IS NOT NULL),
so NULL allows multiple rows but empty string '' does not — every blank-email
customer after the first was hitting a duplicate-key violation at save time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Infrastructure/Services/CsvImportService.cs

@@ -480,7 +480,12 @@ public class CsvImportService : ICsvImportService
                 {
                     // Strip any literal quote characters that QB/Excel may wrap around field values
                     var cleanCompanyName = StripQuotes(record.CompanyName);
-                    var cleanEmail = StripQuotes(record.Email);
+                    // Normalise to null (not empty string) — the UNIQUE index on (CompanyId, Email)
+                    // uses HasFilter("[Email] IS NOT NULL"), so NULL is allowed for multiple rows
+                    // but "" (empty string) is not NULL and would cause a unique-constraint violation
+                    // on the second blank-email customer saved.
+                    var rawEmail   = StripQuotes(record.Email);
+                    var cleanEmail = string.IsNullOrWhiteSpace(rawEmail) ? null : rawEmail;
                     var firstName = StripQuotes(record.ContactFirstName)?.Trim();
                     var lastName = StripQuotes(record.ContactLastName)?.Trim();