Return an empty network list if nothing matches filter

Initializing the network list struct in order to return an empty list
instead of a nil object.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>
(cherry picked from commit a7043ac5c423cf68d4f92024e694887099640d91)

api/server/router/network/filter.go

@@ -53,7 +53,7 @@ func filterNetworks(nws []types.NetworkResource, filter filters.Args) ([]types.N
 		return nil, err
 	}
 
-	var displayNet []types.NetworkResource
+	displayNet := []types.NetworkResource{}
 	for _, nw := range nws {
 		if filter.Include("driver") {
 			if !filter.ExactMatch("driver", nw.Driver) {