Resolve "Streamline: reduce optimize_max_exposure_time for QRCODE_NOT_READABLE"

Closes #48 (closed)

When no QR-code can be read it is most likely there is no sample. In that case:

• maximum exposure time is the requested count_time (most often 1 sec because you call it like streamline_scanner.run(1))
• do not run per-sample exposure condition optimization (if enabled)