Remove redunant commas of tail null values

Certain compilers require null value assignments to end with commas.
The `Namelist` writer contains a special rule to append this comma.

If `end_comma` is enabled, which always finalizes values with commas,
then this would append a second comma, which would transform the record
into two null values.

This patch prevents the redundant comma if end_commas are enabled.

Thanks to Paul Leopardi (@penguian) for reporting.

Author: marshallward

f90nml/namelist.py

@@ -914,7 +914,8 @@ def _var_strings(self, v_name, v_values, v_idx=None, v_start=None):
                 val_strs.append(val_line.rstrip())
 
             # Final null values must always precede a comma
-            if val_strs and (len(v_values) == 0 or v_values[-1] is None):
+            if (val_strs and (len(v_values) == 0 or v_values[-1] is None)
+                    and not self.end_comma):
                 # NOTE: val_strs has been rstrip-ed so lead with a space
                 val_strs[-1] += ' ,'
 

tests/null_target_end_comma.nml

@@ -0,0 +1,12 @@
+&null_nml
+    null_value = ,
+/
+
+&null_comma_nml
+    null_comma = ,
+/
+
+&null_nocomma_rpt_nml
+    null_one = ,
+    null_two = ,
+/

tests/test_f90nml.py

@@ -597,6 +597,12 @@ def test_null(self):
         self.assertEqual(self.null_nml, test_nml)
         self.assert_write(test_nml, 'null_target.nml')
 
+    def test_null_end_comma(self):
+        test_nml = f90nml.read('null.nml')
+        test_nml.end_comma = True
+        self.assertEqual(self.null_nml, test_nml)
+        self.assert_write(test_nml, 'null_target_end_comma.nml')
+
     def test_unset(self):
         test_nml = f90nml.read('unset.nml')
         self.assertEqual(self.unset_nml, test_nml)