builtins-utils, dir: Be more strict about what is acceptable

The 'arch' parameter of flatpak_dir_remote_fetch_indexed_summary() is
used to construct the names of the caches, and can't be NULL.  This
function is used by flatpak_remote_state_ensure_subsummary(), which is
used by ensure_remote_state_arch().  So, the parameter can't be NULL in
those either.
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