fix(schemas): accept empty-string fingerprint on externalProxy

The External Proxy form offers a "Default" option with value '' for the
uTLS fingerprint dropdown, but UtlsFingerprintSchema.optional() rejects
empty strings (only undefined or a valid enum member). Saving an inbound
with externalProxy rows failed with `expected one of "360"|"chrome"|...`.

Preprocess '' to undefined before the optional enum, matching the existing
pattern used for VmessSecuritySchema.

frontend/src/schemas/protocols/stream/external-proxy.ts

@@ -17,7 +17,10 @@ export const ExternalProxyEntrySchema = z.object({
   port: PortSchema.default(443),
   remark: z.string().default(''),
   sni: z.string().optional(),
-  fingerprint: UtlsFingerprintSchema.optional(),
+  fingerprint: z.preprocess(
+    (val) => (val === '' ? undefined : val),
+    UtlsFingerprintSchema.optional(),
+  ),
   alpn: z.array(AlpnSchema).optional(),
 });
 export type ExternalProxyEntry = z.infer<typeof ExternalProxyEntrySchema>;