fix: address remaining CodeRabbit comments — file_create key types, nullable fields, misleading comment

Signed-off-by: Mounil Kanakhara <mounilkankhara@gmail.com>

Author: Mounil2005

src/hiero_sdk_python/file/file_create_transaction.py

@@ -3,6 +3,7 @@
 import time
 
 from hiero_sdk_python.channels import _Channel
+from hiero_sdk_python.crypto.key import Key
 from hiero_sdk_python.crypto.public_key import PublicKey
 from hiero_sdk_python.executable import _Method
 from hiero_sdk_python.hapi.services import file_create_pb2
@@ -199,10 +200,10 @@ def _from_proto(self, proto: file_create_pb2.FileCreateTransactionBody) -> FileC
         Returns:
             FileCreateTransaction: This transaction instance.
         """
-        self.keys = [PublicKey._from_proto(key) for key in proto.keys.keys] if proto.keys.keys else []
-        self.contents = proto.contents
+        self.keys = [Key.from_proto_key(key) for key in proto.keys.keys] if proto.keys.keys else []
+        self.contents = proto.contents or None
         self.expiration_time = (
             Timestamp._from_protobuf(proto.expirationTime) if proto.HasField("expirationTime") else None
         )
-        self.file_memo = proto.memo
+        self.file_memo = proto.memo or None
         return self

src/hiero_sdk_python/transaction/transaction.py

@@ -123,7 +123,7 @@ def __init__(self) -> None:
         # This allows us to maintain the signatures for each unique transaction
         # and ensures that the correct signatures are used when submitting transactions
         self._signature_map: dict[bytes, basic_types_pb2.SignatureMap] = {}
-        # changed from int: 2_000_000 to Hbar: 0.02
+        # changed from int: 2_000_000 to Hbar: 2
         self._default_transaction_fee = Hbar(2)
         self.operator_account_id = None
         self.batch_key: Key | None = None

tests/unit/file_create_transaction_test.py

@@ -259,8 +259,8 @@ def test_file_create_transaction_from_proto():
     from_proto = FileCreateTransaction()._from_proto(file_create_pb2.FileCreateTransactionBody())
 
     # Verify empty protobuf deserializes to empty/default values
-    assert from_proto.contents == b""
-    assert from_proto.file_memo == ""
+    assert from_proto.contents is None
+    assert from_proto.file_memo is None
     assert from_proto.keys == []