is-empty conditions should accept missing values

Returning False from is-empty condition on missing values seems extremely
confusing and rules out some potential use cases.

Closes-Bug: #1578184
Change-Id: I8f976516f89367512e2ffae2815085be1776b6f6

ironic_inspector/plugins/rules.py

@@ -69,6 +69,7 @@ class NeCondition(SimpleCondition):
 
 class EmptyCondition(base.RuleConditionPlugin):
     REQUIRED_PARAMS = set()
+    ALLOW_NONE = True
 
     def check(self, node_info, field, params, **kwargs):
         return field in ('', None, [], {})

ironic_inspector/test/functional.py

@@ -320,6 +320,7 @@ class Test(Base):
                     {'field': 'inventory.interfaces[*].ipv4_address',
                      'op': 'contains', 'value': r'127\.0\.0\.1',
                      'invert': True, 'multiple': 'all'},
+                    {'field': 'i.do.not.exist', 'op': 'is-empty'},
                 ],
                 'actions': [
                     {'action': 'set-attribute', 'path': '/extra/foo',

releasenotes/notes/is-empty-missing-a590d580cb62761d.yaml

@@ -0,0 +1,3 @@
+---
+fixes:
+  - Fixed the "is-empty" condition to return True on missing values.