Fix PKCS7 export quirks.

Fix PKCS7 export quirks.

On macOS, exporting an empty X509Certificate2Collection to PKCS7 would
produce an exception. Instead, we return a pre-encoded empty PKCS7
result.

On Windows, the PKCS7 export would include trailing zeros in PKCS7
export. This occured because the first calls to CertSaveStore would
return a size that is at least big enough, but not exact. This trims the
result to the actual amount written on the second call.