Fix non mandatory output issue in DWIBiasCorrect

The function `_list_outputs` in `DWIBiasCorrect` doesn't take into account the fact that `output_file` is not a mandatory input, thus making any pipeline using this interface without specifying the output crash.

Author: MatthieuJoulot

nipype/interfaces/mrtrix3/preprocess.py

@@ -256,11 +256,12 @@ def _format_arg(self, name, trait_spec, value):
         return super()._format_arg(name, trait_spec, value)
 
     def _list_outputs(self):
-        outputs = self.output_spec().get()
-        outputs["out_file"] = op.abspath(self.inputs.out_file)
-        if self.inputs.bias:
-            outputs["bias"] = op.abspath(self.inputs.bias)
-        return outputs
+        if self.inputs.out_file:
+            outputs = self.output_spec().get()
+            outputs["out_file"] = op.abspath(self.inputs.out_file)
+            if self.inputs.bias:
+                outputs["bias"] = op.abspath(self.inputs.bias)
+            return outputs
 
 
 class DWIPreprocInputSpec(MRTrix3BaseInputSpec):