fix: coerce null _id to empty string in revoke fingerprint match (v0.1.7)

a key doc with an explicit `_id: null` made doc.get("_id", "") return None
instead of the default, so .startswith(prefix) raised an uncaught
AttributeError instead of the contracted clean [✘] + exit 1.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/envelope_authorizer/__init__.py

@@ -1 +1 @@
-__version__ = "0.1.6"
+__version__ = "0.1.7"

src/envelope_authorizer/commands/revoke.py

@@ -23,7 +23,7 @@ def _find_by_fingerprint(storage, prefix: str):
     """
     if not prefix:
         raise CommandError("fingerprint prefix must not be empty")
-    matches = [doc for doc in storage.get_all() if doc.get("_id", "").startswith(prefix)]
+    matches = [doc for doc in storage.get_all() if (doc.get("_id") or "").startswith(prefix)]
     if len(matches) > 1:
         ids = ", ".join(d["_id"][:16] for d in matches)
         raise CommandError(f"fingerprint prefix '{prefix}' is ambiguous; matches: {ids}")