fix(authn-jwt): avoid to save empty claim name

An empty claim_name has not a real meaning and will result in a
syntax error cluster_override.conf. i.e:
```
authentication {
  mechanism = "jwt"
  verify_claims { = "22"}
  ...
}
```

apps/emqx_authn/src/simple_authn/emqx_authn_jwt.erl

@@ -441,6 +441,11 @@ check_claim_name(iat) ->
     false;
 check_claim_name(nbf) ->
     false;
+check_claim_name(Name) when
+    Name == <<>>;
+    Name == ""
+->
+    false;
 check_claim_name(_) ->
     true.