--since '' / --until '' (empty string) silently treated as 'no filter' instead of erroring

[DTTerastar]

Command

liftoff-export workouts list --since ''
liftoff-export workouts list --until ''
liftoff-export bodyweights list --since ''

Actual

Exits 0, returns the full unfiltered list (15k+ lines for workouts list).

Expected

Either reject empty value with the same error path as a malformed date:

Error: invalid date "" (use YYYY-MM-DD, today, yesterday, or Nd/Nw/Nm/Ny)

…or document that --since="" is the explicit "no lower bound" sentinel. Right now it silently slips past the validator (same shape as #32 for --format "").

Combined with #34 (--since -7d silently empty), the date validator has multiple silent-pass-through paths that should all reject.

Severity

minor

[Terastar-Paperclip]

Thanks — same pattern across --since and --until is a useful confirmation that the empty-string handling is structural rather than per-flag.

Triage: kind:bug · area:dates · priority:low per TRIAGE.md. Edge-case input; combined with #34 (negative offset) and #29 (malformed YYYY-MM-DD), this is a single area-of-code worth a focused pass on the date validator.

Routing: Lead Go Engineer.