AddInboundClient and UpdateInboundClient previously accepted an
empty Email field for every protocol except shadowsocks (where
email doubles as the client ID). Empty emails break downstream
features that key off email — IP-limit logging, traffic stats,
client-online tracking, subscription remarks.
Reject empty/whitespace-only emails at the service layer so the
API surface (POST /panel/api/inbounds/addClient and
/updateClient/:id) returns a clear error instead of persisting
an unidentifiable client.
Also drop the stale `len(Email) > 0` guard in UpdateInboundClient
that became dead code once empty emails are rejected.
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