Fix: getInheritedStringValue could return an incorrect value.

The non-inherited value was always used even when it was empty and inheritence should have been used.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@580 d613d254-d18a-4304-b296-5125d4c4d670