new API: make sure targets in Targets can be empty

This change is relevant to the new metadata class Targets.

In the specification, when describing the Targets metadata file format
and more precisely "TARGETPATH" (or targets containing the actual
target files) it's said:
"It is allowed to have a TARGETS object with no TARGETPATH elements.
This can be used to indicate that no target files are available."

If there is no "TARGETPATH" keys for the dictionary "targets", this
would mean that "Targets.targets" is {}.
Make sure we test for that.

See: https://theupdateframework.github.io/specification/latest/#targetpath

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tests/test_api.py

@@ -585,6 +585,13 @@ def test_metadata_targets(self):
             targets.signed.targets[filename].to_dict(), fileinfo.to_dict()
         )
 
+        # Test from_dict/to_dict Targets with empty targets.
+        targets_dict = copy.deepcopy(targets.to_dict())
+        targets_dict["signed"]["targets"] = {}
+        tmp_dict = copy.deepcopy(targets_dict["signed"])
+        targets_obj = Targets.from_dict(tmp_dict)
+        self.assertEqual(targets_dict["signed"], targets_obj.to_dict())
+
         # Test from_dict/to_dict Targets without delegations
         targets_dict = targets.to_dict()
         del targets_dict["signed"]["delegations"]