fix: guard against None app ID before JWT creation

Addresses review feedback — str(None) would produce 'None' as the
issuer, which passes PyJWT's type check but gets rejected by GitHub.
Raise GitHubAppError with a clear message instead.

Author: primetheus

src/githubapp/core.py

@@ -577,6 +577,12 @@ def _create_jwt(self, expiration=60):
         :param expiration: int
         :return string:
         """
+        if self.id is None:
+            raise GitHubAppError(
+                message="GitHub App ID (github_app_id) is not configured; cannot generate JWT.",
+                status=None,
+                data=None,
+            )
         now = int(time.time())
         payload = {"iat": now, "exp": now + expiration, "iss": str(self.id)}
         encrypted = jwt.encode(payload, key=self.key, algorithm="RS256")