Add null value support to BaseEmailDocument interface

- Update BaseEmailDocument to support `| null` for optional fields (cc, bcc, from, replyTo, text, etc.)
- Update BaseEmailTemplateDocument to support `| null` for optional fields
- Add explicit null checks in sendEmail processing to handle null values properly
- Update CUSTOM-TYPES.md documentation to reflect null value compatibility

Fixes type constraint error where customer Email types had `cc?: string[] | null`
but BaseEmailDocument only supported `cc?: string[]`.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/sendEmail.ts

@@ -83,10 +83,10 @@ export const sendEmail = async <TEmail extends BaseEmailDocument = BaseEmailDocu
   if (emailData.to) {
     emailData.to = parseAndValidateEmails(emailData.to as string | string[])
   }
-  if (emailData.cc) {
+  if (emailData.cc && emailData.cc !== null) {
     emailData.cc = parseAndValidateEmails(emailData.cc as string | string[])
   }
-  if (emailData.bcc) {
+  if (emailData.bcc && emailData.bcc !== null) {
     emailData.bcc = parseAndValidateEmails(emailData.bcc as string | string[])
   }