Fix DefaultOvsdbClient Class's getBridges's improper return value

When tableStore or rowStore is null, getBridges() should not return (null),
that will cause NullPointerException in its two usages:

(getBRidges only has these two usages now, but it is very foundational.)

1. OvsdbBridgeConfig.java, line 113-115:

        Set<OvsdbBridge> bridges = client.getBridges();
        return bridges.stream()......

2. OvsdbControllerConfig.java, line 92:

	.filter(cs ->
	 cs.getBridges().stream()
	.anyMatch(b -> dpidMatches(b, ofDeviceId)))
	......

Change-Id: Ice14a8991f5022a5041cf48f5c8431712f946228

@@ -923,11 +923,11 @@ public class DefaultOvsdbClient implements OvsdbProviderService, OvsdbClientServ
         Set<OvsdbBridge> ovsdbBridges = new HashSet<>();
         OvsdbTableStore tableStore = getTableStore(DATABASENAME);
         if (tableStore == null) {
-            return null;
+            return ovsdbBridges;
         }
         OvsdbRowStore rowStore = tableStore.getRows(BRIDGE);
         if (rowStore == null) {
-            return null;
+            return ovsdbBridges;
         }
         ConcurrentMap<String, Row> rows = rowStore.getRowStore();
         for (String uuid : rows.keySet()) {