msi: WriteRegistryData advoid NULL data

[Aric Stewart]

Win9x RegSetValueExW does not deal with NULL data. so make the data an 
empty string instead of null
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Index: dlls/msi/action.c
===================================================================
RCS file: /home/wine/wine/dlls/msi/action.c,v
retrieving revision 1.102
diff -u -u -r1.102 action.c
--- dlls/msi/action.c	16 Mar 2005 11:31:35 -0000	1.102
+++ dlls/msi/action.c	21 Mar 2005 15:24:49 -0000
@@ -3523,7 +3523,8 @@
             value_data = parse_value(package, value, &type, &size); 
         else
         {
-            value_data = NULL;
+            static const WCHAR szEmpty[] = {0};
+            value_data = szEmpty;
             size = 0;
             type = REG_SZ;
         }