pmb.parse.apkindex: fix crash when origin or timestamp are unset (MR 2449)

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`
[ci:skip-build]: already built successfully in CI