betterproto: avoid assigning a var of type Optional[InclusionPromise] to TransparencyLogEntry.inclusion_promise

```
  File "/usr/local/google/home/rpetgrave/src/ssp/sigstore-python/sigstore/models.py", line 262, in _to_rekor
    tlog_entry = rekor_v1.TransparencyLogEntry(
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/google/home/rpetgrave/src/ssp/.venv/lib/python3.12/site-packages/pydantic/_internal/_dataclasses.py", line 120, in __init__
    s.__pydantic_validator__.validate_python(ArgsKwargs(args, kwargs), self_instance=s)
pydantic_core._pydantic_core.ValidationError: 1 validation error for TransparencyLogEntry
inclusion_promise
  Input should be a dictionary or an instance of InclusionPromise [type=dataclass_type, input_value=None, input_type=NoneType]
```

Signed-off-by: Ramon Petgrave <[email protected]>

Author: ramonpetgrave64

CHANGELOG.md

@@ -10,6 +10,8 @@ All versions prior to 0.9.0 are untracked.
 
 ### Fixed
 
+* Avoid pydantic's instantiation issues with `TransparencyLogEntry` when `InclusionPromise` is not present.
+
 * TSA: Changed the Timestamp Authority requests to explicitly use sha256 for message digests.
   [#1373](https://github.com/sigstore/sigstore-python/pull/1373)
 

sigstore/models.py

@@ -239,12 +239,6 @@ def _to_rekor(self) -> rekor_v1.TransparencyLogEntry:
 
         @private
         """
-        inclusion_promise: rekor_v1.InclusionPromise | None = None
-        if self.inclusion_promise:
-            inclusion_promise = rekor_v1.InclusionPromise(
-                signed_entry_timestamp=base64.b64decode(self.inclusion_promise)
-            )
-
         inclusion_proof = rekor_v1.InclusionProof(
             log_index=self.inclusion_proof.log_index,
             root_hash=bytes.fromhex(self.inclusion_proof.root_hash),
@@ -257,10 +251,14 @@ def _to_rekor(self) -> rekor_v1.TransparencyLogEntry:
             log_index=self.log_index,
             log_id=common_v1.LogId(key_id=bytes.fromhex(self.log_id)),
             integrated_time=self.integrated_time,
-            inclusion_promise=inclusion_promise,  # type: ignore[arg-type]
             inclusion_proof=inclusion_proof,
             canonicalized_body=base64.b64decode(self.body),
         )
+        if self.inclusion_promise:
+            inclusion_promise = rekor_v1.InclusionPromise(
+                signed_entry_timestamp=base64.b64decode(self.inclusion_promise)
+            )
+            tlog_entry.inclusion_promise = inclusion_promise
 
         # Fill in the appropriate kind
         body_entry: ProposedEntry = TypeAdapter(ProposedEntry).validate_json(