Fix retrieval of object file when destination is /dev/null

Fix retrieval of object file when destination is /dev/null

ResultRetriever::on_entry_data assumes that a destination file has been
opened if the entry type is not stderr_output, but that’s incorrect
since on_entry_start doesn’t open a destination file if it’s /dev/null.
An assertion is triggered:

    ccache: ResultRetriever.cpp:129: virtual void
    ResultRetriever::on_entry_data(const uint8_t *, size_t): failed
    assertion: (m_dest_file_type == FileType::stderr_output &&
    !m_dest_fd) || (m_dest_file_type != FileType::stderr_output &&
    m_dest_fd)

Fix this by letting on_entry_data handle the “destination file not
opened” case and correcting the assert.

Fixes #752.