Fix bypass list length check when all values are None

The patch https://review.openstack.org/#/c/527001/ bypassed the None
list but did not consider the case where all the for_each lists are
getting their values from attributes and therefore are None.

This patch set fix it by also skipping the check if all the values
are None -- as len(value_lens) would be 0 in that case.

Closes-bug: #1732934
Change-Id: Iba574bbd3877f9ca1ca755c384b3ddb47aec711c

heat/engine/hot/functions.py

@@ -956,7 +956,7 @@ class Repeat(function.Function):
                 self._valid_arg(value)
                 values.append(value)
                 value_lens.append(len(value))
-        if not self._nested_loop:
+        if not self._nested_loop and value_lens:
             if len(set(value_lens)) != 1:
                 raise ValueError(_('For %s, the length of for_each values '
                                    'should be equal if no nested '

heat/tests/test_hot.py

@@ -1109,7 +1109,17 @@ class HOTemplateTest(common.HeatTestCase):
         snippet = {'map_merge': {'repeat': {
             'template': {'ROLE': 'ROLE', 'NAME': 'NAME'},
             'for_each': {'ROLE': None, 'NAME': ['n1', 'n2']},
-            'permutations': None}}}
+            'permutations': False}}}
+        tmpl = template.Template(hot_mitaka_tpl_empty)
+        resolved = self.resolve(snippet, tmpl)
+
+        self.assertEqual({}, resolved)
+
+    def test_merge_containing_repeat_multi_list_no_nested_loop_all_none(self):
+        snippet = {'map_merge': {'repeat': {
+            'template': {'ROLE': 'ROLE', 'NAME': 'NAME'},
+            'for_each': {'ROLE': None, 'NAME': None},
+            'permutations': False}}}
         tmpl = template.Template(hot_mitaka_tpl_empty)
         resolved = self.resolve(snippet, tmpl)