fix(schema): accept null location in position validation (#163)

Positions with no location set store `location: null` in the frontend.
When the skill-link toggle sends the full position back to the API,
Zod's `.optional()` rejects null (it only accepts undefined), causing
a 400 ValidationError. Add `.nullable()` to locationSchema so both
null and undefined are accepted.

src/routes/import.ts

···
   location:
     | string
     | { country: string; region?: string; city?: string; countryCode?: string }
+    | null
     | undefined,
 ): { country: string; region?: string; city?: string; countryCode?: string } | undefined {
   if (!location) return undefined;

src/routes/schemas.ts

···
     }),
     z.string(),
   ])
+  .nullable()
   .optional();
 
 export const profileSelfSchema = z