fix: check targetId before unregistering the connector

When the handshake has failed, the targetId is not defined, and the
removal was throwing a NullPointerException.

gravitee-exchange-connector/gravitee-exchange-connector-core/src/main/java/io/gravitee/exchange/connector/core/DefaultExchangeConnectorManager.java

@@ -68,7 +68,9 @@ public Completable register(final ExchangeConnector exchangeConnector) {
     public Completable unregister(final ExchangeConnector exchangeConnector) {
         return Completable
             .defer(() -> {
-                exchangeConnectors.remove(exchangeConnector.targetId(), exchangeConnector);
+                if (exchangeConnector.targetId() != null) {
+                    exchangeConnectors.remove(exchangeConnector.targetId(), exchangeConnector);
+                }
                 return exchangeConnector.close();
             })
             .doOnComplete(() -> log.debug("Connector successfully unregister for target [{}]", exchangeConnector.targetId()))