FIX: Default value for sbatch_args (SLURMGraph)

Added a default value for self._sbatch_args in the SLURMGraph Plugin.
This was not present and caused nipype to abort when sbatch_args was not specified when running the plugin.

Author: brennerd11

nipype/pipeline/plugins/slurmgraph.py

@@ -55,6 +55,8 @@ def __init__(self, **kwargs):
                     self._template = open(self._template).read()
             if 'sbatch_args' in kwargs['plugin_args']:
                 self._sbatch_args = kwargs['plugin_args']['sbatch_args']
+            else:  # default argument for _sbatch_args
+                self._sbatch_args = ''
             if 'dont_resubmit_completed_jobs' in kwargs['plugin_args']:
                 self._dont_resubmit_completed_jobs = kwargs['plugin_args'][
                     'dont_resubmit_completed_jobs']