EHdr: implements GetNoDataValue() instead of calling SetNoDataValue() in the opening code, so as to avoid to rely on PAM to return the nodata value (since the nodata primarily comes from the .hdr file and not PAM)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25701 f0d54148-0727-0410-94bb-9a71ac55c965