STYLE: Avoid warning with divide by zero

A warning was displayed when the length of an iterable was zero.
"Avoid RuntimeWarning: divide by zero encountered in log10"

Avoid this warning by explicitly checking for zero and giving
alternate behavior when no nodes are presented.

Author: hjmjohnson

nipype/pipeline/engine.py

@@ -750,7 +750,14 @@ def _write_report_info(self, workingdir, name, graph):
                                            value=1))
         save_json(graph_file, json_dict)
         graph_file = op.join(report_dir, 'graph.json')
-        template = '%%0%dd_' % np.ceil(np.log10(len(nodes))).astype(int)
+        # Avoid RuntimeWarning: divide by zero encountered in log10
+        num_nodes = len(nodes)
+        if num_nodes > 0:
+            index_name = np.ceil(np.log10(num_nodes)).astype(int)
+        else:
+            index_name = 0
+        template = '%%0%dd_' % index_name
+
         def getname(u, i):
             name_parts = u.fullname.split('.')
             #return '.'.join(name_parts[:-1] + [template % i + name_parts[-1]])