fixed some logic for linkages when units are None

dymos/trajectory/trajectory.py

@@ -597,13 +597,17 @@ def _update_linkage_options_configure(self, linkage_options):
         if linkage_options['units_b'] is _unspecified:
             linkage_options['units_b'] = units['b']
 
-        conversion_scaler, conversion_offset = unit_conversion(linkage_options['units_a'], linkage_options['units_b'])
-
-        if not linkage_options['connected'] and linkage_options['units'] is _unspecified and \
-            (abs(conversion_scaler - 1.0) > 1.0E-15 or abs(conversion_offset) > 1.0E-15):
-                raise ValueError(f'{info_str}Linkage units were not specified but the units of {phase_name_a}.{var_a} '
-                                 f'({units["a"]}) and {phase_name_b}.{var_b} ({units["b"]}) are not equivalent. '
-                                 f'Units for this linkage constraint must be specified explicitly.')
+        if units['a'] is not None and units['b'] is not None:
+            conversion_scaler, conversion_offset = unit_conversion(units['a'], units['b'])
+        else:
+            conversion_scaler, conversion_offset = (1.0, 0.0)
+
+        if not linkage_options['connected'] \
+                and linkage_options['units'] is _unspecified \
+                and (abs(conversion_scaler - 1.0) > 1.0E-15 or abs(conversion_offset) > 1.0E-15):
+            raise ValueError(f'{info_str}Linkage units were not specified but the units of {phase_name_a}.{var_a} '
+                             f'({units["a"]}) and {phase_name_b}.{var_b} ({units["b"]}) are not equivalent. '
+                             f'Units for this linkage constraint must be specified explicitly.')
         else:
             linkage_options['units'] = units['a']