The `race` condition directive was broken due to
a strict dependency on `threads > 0` for parallel
execution, causing templates with `race` directive
enabled but no explicit threads to fall back to
seq execution.
This regression was introduced in v3.2.0 (#4868),
which restricted parallel execution to only when
`payloads` were present.
Fixes#5713 to allow race conditions even w/o
explicit `payloads`, and add a default thread
count when race is enabled but threads is 0.
Signed-off-by: Dwi Siswanto <git@dw1.io>