`--tp-ignore-env` sends zeros rather than removing the parent_id

[mterhar]

When I tried to use this argument with a traceparent set, overwrite the parent span ID with zeros but it sent them as a string somehow.

Script in question:

    END_TIME=$(date +%s.%N)
 
    otel-cli span \
        --name "cron-deploy" \
        --start "$START_TIME" \
        --end "$END_TIME" \
        --force-trace-id "$OTEL_CLI_FORCE_TRACE_ID" \
        --force-span-id "$ROOT_SPAN_ID" \
        --tp-ignore-env \
        --attrs "script=run-update.sh,status=$FINAL_STATUS,script_used=$SCRIPT_USED"

I fixed it by just removing the traceparent header all the way.

    END_TIME=$(date +%s.%N)
    unset TRACEPARENT
    otel-cli span \
        --name "cron-deploy" \
        --start "$START_TIME" \
        --end "$END_TIME" \
        --force-trace-id "$OTEL_CLI_FORCE_TRACE_ID" \
        --force-span-id "$ROOT_SPAN_ID" \
        --attrs "script=run-update.sh,status=$FINAL_STATUS,script_used=$SCRIPT_USED"

[tobert]

Fix is up in PR #25. The root cause: LoadTraceparent() with --tp-ignore-env returns a traceparent with Initialized=true but zeroed SpanId. The code unconditionally set ParentSpanId = tp.SpanId, producing a span with an all-zeros parent. Now guarded with bytes.Equal(tp.SpanId, GetEmptySpanId()). -Claude and @tobert