viz of DAG not implemented if no targets

When using the ```-vd```option without targets, the program runs normally and says the image was created although this is not the case:

```python -m predator networks/ns_2.sbml -vd viz/ns_2_dag.png```

gives the following output:
```
DEBUG args = Namespace(export=None, forbidden_seeds=[], forbidden_seeds_file=None, greedy=False, infile='networks/ns_2.sbml', info=False, intersection=False, no_prerun=False, no_topological_injection=False, pareto=False, pareto_full=False, scc_with_asp=False, seeds=[], seeds_file=None, semantic_injection=False, targets=[], targets_are_forbidden=False, targets_file=None, union=False, verbose=False, visualize=None, visualize_dag='viz/ns_2_dag.png', visualize_without_reactions=None)
Solution 1:
h, o, a, g, e, k, m

Solution 2:
h, n, a, g, e, k, m

Solution 3:
h, a, g, e, k, m, b

end of solutions.
Rendering…
-> DAG of SCC rendered in viz/ns_2_dag.png
-> Ok!
TIME DATA EXTRACTION: 0.0s
TIME PRE-RUN : 0.334s
TIME SCCs EXTRACTION: unperformed
TIME SEED SEARCH : 2.035s
TIME RENDERING : 0.0s
TIME TOTAL : 2.036s
```

The reason is that without targets, the DAG is not generated. Since args.visualize_dag is replaced by the dag only if targets exist, the code exits normally as if the graph was exported and the output says so.