fix(axon): reject requests with missing signature in default_verify

default_verify gated the signature check behind
`if synapse.dendrite.signature`, so a request with an empty or absent
signature skipped verification entirely. This lets anyone impersonate
any dendrite hotkey (e.g. a high-stake validator) by simply omitting the
signature — the request then passes verify and reaches blacklist/forward
as if it were genuinely signed.

Require a signature to be present, mirroring the existing "Missing Nonce"
guard, then verify it. A present-but-invalid signature was already
rejected; this closes the empty-signature hole.