dt_imageio_open_tiff(): use TIFFGetFieldDefaulted to get TIFFTAG_SAMPLEFORMAT

Some applications(e.g. convert(1)) do not set it, and using TIFFGetFieldDefaulted ensures that it we get some value.
Apparently default is SAMPLEFORMAT_UINT.
If there is an app that does not set this field to SAMPLEFORMAT_IEEEFP when saving float data there, or otherwise, there might be problems...