[raster] ST_BandMetadata should not report hasnodata anymore

[pracine]

Now that the absence of nodata value is generally reported as NULL (as in ST_BandNodataValue()), and even if there is an internal flag that is still necessary, there is not need to return a hasnodata column when invoking ST_BandMetadata(). Just return NULL when the flag is false.

The report is confusing as it can say FALSE for hasnodata and still report what is set as nodata value even if it is not used. The expected behaviour is to report NULL when the flag is FALSE.

[dustymugs]

Fixed in r9484.