Metadata API: Fix DelegatedRole serialization issue

A DelegatedRole with paths=[] fails to serialize correctly (paths is not
included in the output json).

Fix the issue, modify tests to notice a regression.

Fixes #1389

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tests/test_api.py

@@ -451,21 +451,23 @@ def test_delegated_role_class(self):
             with self.assertRaises(ValueError):
                 DelegatedRole.from_dict(role.copy())
 
-            # Test creating DelegatedRole only with "path_hash_prefixes"
+            # Test creating DelegatedRole only with "path_hash_prefixes" (an empty one)
             del role["paths"]
-            DelegatedRole.from_dict(role.copy())
-            role["paths"] = "foo"
+            role["path_hash_prefixes"] = []
+            role_obj = DelegatedRole.from_dict(role.copy())
+            self.assertEqual(role_obj.to_dict(), role)
 
-            # Test creating DelegatedRole only with "paths"
+            # Test creating DelegatedRole only with "paths" (now an empty one)
             del role["path_hash_prefixes"]
-            DelegatedRole.from_dict(role.copy())
-            role["path_hash_prefixes"] = "foo"
+            role["paths"] = []
+            role_obj = DelegatedRole.from_dict(role.copy())
+            self.assertEqual(role_obj.to_dict(), role)
 
             # Test creating DelegatedRole without "paths" and
             # "path_hash_prefixes" set
             del role["paths"]
-            del role["path_hash_prefixes"]
-            DelegatedRole.from_dict(role)
+            role_obj = DelegatedRole.from_dict(role.copy())
+            self.assertEqual(role_obj.to_dict(), role)
 
 
     def test_delegation_class(self):

tuf/api/metadata.py

@@ -768,7 +768,7 @@ def __init__(
         super().__init__(keyids, threshold, unrecognized_fields)
         self.name = name
         self.terminating = terminating
-        if paths and path_hash_prefixes:
+        if paths is not None and path_hash_prefixes is not None:
             raise ValueError(
                 "Only one of the attributes 'paths' and"
                 "'path_hash_prefixes' can be set!"
@@ -804,9 +804,9 @@ def to_dict(self) -> Dict[str, Any]:
             "terminating": self.terminating,
             **base_role_dict,
         }
-        if self.paths:
+        if self.paths is not None:
             res_dict["paths"] = self.paths
-        elif self.path_hash_prefixes:
+        elif self.path_hash_prefixes is not None:
             res_dict["path_hash_prefixes"] = self.path_hash_prefixes
         return res_dict