Add some extra checks for null strings.

[David Hampton]

Log Message:
-----------
Add some extra checks for null strings.

Tags:
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gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/app-utils:
        global-options.c
    gnucash/src/core-utils:
        gnc-gconf-utils.c
    gnucash/src/gnome-utils:
        dialog-preferences.c
        dialog-utils.c