d2d pipeline fails if there are NO Java source file on the from side inputs

[pombredanne]

not an automaton yet; add some words and call make_automaton

Traceback:
  File "/opt/scancodeio/scanpipe/pipelines/__init__.py", line 120, in execute
    step(self)
  File "/opt/scancodeio/scanpipe/pipelines/deploy_to_develop.py", line 137, in map_java_to_class
    d2d.map_java_to_class(project=self.project, logger=self.log)
  File "/opt/scancodeio/scanpipe/pipes/d2d.py", line 246, in map_java_to_class
    _map_java_to_class_resource(to_resource, from_resources, from_classes_index)
  File "/opt/scancodeio/scanpipe/pipes/d2d.py", line 191, in _map_java_to_class_resource
    match = pathmap.find_paths(path=normalized_java_path, index=from_classes_index)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/scancodeio/scanpipe/pipes/pathmap.py", line 84, in find_paths
    matches = list(index.iter_long(reversed_path))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^