Handle this undocumented case where gconf_client_set_string() can return

[David Hampton]

Log Message:
-----------
Handle this undocumented case where gconf_client_set_string() can
return FALSE without setting the error variable.  Also modify this
function to return NULL if gconf returns an empty string.  This is
more in line with what all the rest of GnuCash code expects.
Centralize error handling in this file.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/core-utils:
        gnc-gconf-utils.c