pmb.parse.apkindex: fix crash when origin or timestamp are unset

Sometimes these fields are not set, e.g.:

pdb> p ret
{'depends': ['build-base', 'bash', 'bison', 'findutils', 'flex', 'musl-dev', 'openssl-dev', 'perl', 'python3'], 'arch': 'noarch', 'version': '20241022.022230', 'pkgname': '.makedepends-linux-lenovo-21bx', 'provides': []}

I hit this crash when I installed the build deps for the linux-lenovo-21bx package in the native chroot, and then ran pmbootstrap stats

pmb/parse/apkindex.py

@@ -94,11 +94,11 @@ def parse_next_block(path: Path, lines: list[str]) -> ApkindexBlock | None:
     return ApkindexBlock(
         arch=Arch.from_str(ret["arch"]),
         depends=ret["depends"],
-        origin=ret["origin"],
+        origin=ret.get("origin"),
         pkgname=ret["pkgname"],
         provides=ret["provides"],
         provider_priority=ret.get("provider_priority"),
-        timestamp=ret["timestamp"],
+        timestamp=ret.get("timestamp"),
         version=ret["version"],
     )