mapi32: Write-strings warnings fix

[Andrew Talbot]

Changelog:
    mapi32: Write-strings warnings fix.

diff -urN a/dlls/mapi32/sendmail.c b/dlls/mapi32/sendmail.c
--- a/dlls/mapi32/sendmail.c	2006-05-23 17:24:40.000000000 +0100
+++ b/dlls/mapi32/sendmail.c	2006-06-03 14:44:21.000000000 +0100
@@ -113,8 +113,8 @@
     }
     if (message->nFileCount) FIXME("Ignoring attachments\n");
 
-    subject = message->lpszSubject ? message->lpszSubject : "";
-    body = message->lpszNoteText ? message->lpszNoteText : "";
+    subject = message->lpszSubject ? message->lpszSubject : NULL;
+    body = message->lpszNoteText ? message->lpszNoteText : NULL;
 
     TRACE( "Subject: %s\n", debugstr_a(subject) );
     TRACE( "Body: %s\n", debugstr_a(body) );