NoDataValue: set max float if undef is given.
Unit: set '' if undef is given.
Also other changes that will also remove some "use of uninitialized value in subroutine entry" warnings.
Automatic handling of SQL result layers.


git-svn-id: https://svn.osgeo.org/gdal/trunk@27301 f0d54148-0727-0410-94bb-9a71ac55c965