[core] command line: do not export an empty list on the command line

but export empty string params on the command line (avoid to switch to
another default value from the command line if not provided).

meshroom/core/node.py

@@ -713,7 +713,8 @@ def _buildAttributeCmdVars(cmdVars, name, attr):
                     cmdVars[name] = '--{name} "{value}"'.format(name=name, value=v)
                     cmdVars[name + 'Value'] = str(v)
 
-                    if v:
+                    # do not export on the command line, if it is an empty list
+                    if not isinstance(attr, ListAttribute) or v:
                         cmdVars[group] = cmdVars.get(group, '') + ' ' + cmdVars[name]
                 elif isinstance(attr, GroupAttribute):
                     assert isinstance(attr.value, DictModel)
@@ -764,7 +765,8 @@ def _buildAttributeCmdVars(cmdVars, name, attr):
             self._cmdVars[name] = '--{name} "{value}"'.format(name=name, value=v)
             self._cmdVars[name + 'Value'] = str(v)
 
-            if v:
+            # do not export on the command line, if it is an empty list
+            if not isinstance(attr, ListAttribute) or v:
                 self._cmdVars[attr.attributeDesc.group] = self._cmdVars.get(attr.attributeDesc.group, '') + \
                                                           ' ' + self._cmdVars[name]