Fix name_source to handle missing source

[effigies]

As discussed in #2450, the name_source logic is not working as expected.

Consider the following input spec:

class NwarpAdjustInputSpec(AFNICommandInputSpec):
    warps = InputMultiPath(
        File(exists=True),
        minlen=5,
        mandatory=True,
        argstr='-nwarp %s',
        desc='List of input 3D warp datasets')
    in_files = InputMultiPath(
        File(exists=True),
        minlen=5,
        argstr='-source %s',
        desc='List of input 3D datasets to be warped by the adjusted warp '
             'datasets.  There must be exactly as many of these datasets as '
             'there are input warps.')
    out_file = File(
        desc='Output mean dataset, only needed if in_files are also given. '
             'The output dataset will be on the common grid shared by the '
             'source datasets.',
        argstr='-prefix %s',
        name_source='in_files',
        name_template='%s_NwarpAdjust',
        keep_extension=True,
        requires=['in_files'])

The goal is to automatically derive an out_file value iff in_files is set. There was some discussion over whether the traits are correct as is (e.g. @oesteban suggested that requires is not needed for out_file), but it did seem to be agreed that the goal should be achievable purely using traits without overriding interface methods.

[effigies]

Figure I should bump this before we get too close to release. Does anybody have time to figure this out?

@oesteban @satra Are there open issues to discuss, or is it a question of implementation?

[oesteban]

I think it is mostly a question of implementation