`igraph_eigenvector_centrality`

has two special cases: it returns all-ones for graphs with no edges or graphs with all-zero weights.

I propose changing this to return all-zeros instead. Reasoning:

- In undirected graphs, isolated vertices have centrality 0. In a graph with no edges, all vertices are isolated.
- In directed graphs with no cycles, 0 is returned for all vertices.
- Currently, if the leading eigenvalue is 0, all-zeros are returned. I can see this in the code. However, I cannot see how the eigenvalue could be zero in the undirected case, unless the graph has no edges (a special case which is caught early). I assume this can only occur due to roundoff errors.
- A common (though not very sophisticated) method to compute eigenvector centralities is to repeatedly multiply by the adjacency matrix and normalize. This would give all-zeros.

I am wondering if I am missing any reasons why it is a good idea to return all-ones. Of course, all-ones are not technically *wrong*. If the graph has no edges, the adjacency matrix contains all zeros, so any vector is an eigenvector.

@Gabor I *think* you implemented this. Do you recall why you chose to return all-ones?

Does anyone else see a reasons why *not* to change to all-zeros?