fix(partners): allow empty-string email fields on partial updates

@IsOptional() only skips validation for null/undefined; an empty string
still trips @IsEmail(). When the operator UI saves billingInfo with a
blank contactEmail, the request 400'd with 'must be an email'. Coerce
'' → undefined on every @IsEmail-decorated field via a shared
@Transform so blank inputs round-trip cleanly.

services/platform-api/src/partners/dto/update-partner.dto.ts

@@ -1,4 +1,4 @@
-import { Type } from 'class-transformer'
+import { Transform, Type } from 'class-transformer'
 import {
   IsEmail,
   IsEnum,
@@ -12,17 +12,25 @@ import {
   ValidateNested,
 } from 'class-validator'
 
+// Coerce '' → undefined so blank form fields don't trip @IsEmail on the
+// nested DTOs. @IsOptional() skips validation only for null/undefined, not
+// empty strings — without this transform a partial edit fails the moment
+// the form contains an unfilled email field.
+const EmptyToUndefined = Transform(({ value }: { value: unknown }) =>
+  value === '' ? undefined : value,
+)
+
 class ContactInfoDto {
   @IsOptional() @IsString() @MaxLength(200) primaryName?: string
-  @IsOptional() @IsEmail() primaryEmail?: string
-  @IsOptional() @IsEmail() billingEmail?: string
+  @EmptyToUndefined @IsOptional() @IsEmail() primaryEmail?: string
+  @EmptyToUndefined @IsOptional() @IsEmail() billingEmail?: string
 }
 
 class BillingInfoDto {
   @IsOptional() @IsString() @MaxLength(200) companyName?: string
   @IsOptional() @IsString() @MaxLength(40) vatId?: string
   @IsOptional() @IsString() @MaxLength(2) country?: string
-  @IsOptional() @IsEmail() contactEmail?: string
+  @EmptyToUndefined @IsOptional() @IsEmail() contactEmail?: string
 }
 
 export class UpdatePartnerDto {