[igraph] clusters does not like empty string node names

[Katharina Zweig]

Dear all,

I just found an interesting behavior of the clusters function which 
might be considered a bug. Here is the minimal code to produce the result:

> v1 <- c("", "Anne", "Bert", "Charlotte")
> v2 <- c("", "Bert", "Charlotte", "Anne")
> df <- data.frame(v1, v2)
> g <- graph.data.frame(df)
> summary(g)
>clusters(g)

As you can see there should be a single node with an empty string name 
and a triangle between the other three nodes but due to the empty string 
name of the first node the  clusters function thinks that all nodes are 
singletons.
It seems to be independent of the order in which the 'empty' node name 
is given.  It  got worsed, when I replaced one of the empty names by a 
real string: my R GUI crashed....

Hope that helps.
Nina