Bug fix in getConnectableBus() in calculated bus branch topology

- method independent of switches' order
- if connectableBus null, return first bus

Signed-off-by: RALAMBOTIANA MIORA <miora.ralambotiana@rte-france.com>

iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/NodeBreakerVoltageLevel.java

@@ -387,14 +387,17 @@ BusExt getConnectableBus(int node) {
             // node associated to a bus
             BusExt[] connectableBus2 = new BusExt[1];
             graph.traverse(node, (v1, e, v2) -> {
+                if (connectableBus2[0] != null) {
+                    return TraverseResult.TERMINATE;
+                }
                 connectableBus2[0] = getBus(v2);
                 if (connectableBus2[0] != null) {
                     return TraverseResult.TERMINATE;
                 }
                 return TraverseResult.CONTINUE;
             });
             // if nothing found, just take the first bus
-            if (connectableBus2[0] != null) {
+            if (connectableBus2[0] == null) {
                 Iterator<CalculatedBus> it = getBuses().iterator();
                 if (!it.hasNext()) {
                     throw new AssertionError("Should not happen");