v2writable: do not clobber {shards} or {parallel} if unset

v2writable: do not clobber {shards} or {parallel} if unset

The $jobs parameter in `public-inbox-convert' is passed to
V2Writable->init_inbox as `undef' by default, causing
parallelization to be disabled.

Instead, leave the underlying {parallel} flag untouched if
$shards is undef and do not clobber the default shard count.

This allows us to take advantage of multicore systems when
running public-inbox-convert with no command-line switches.