Fix #6658, MS08-067 unable to find the right target for W2k3SP0

Fix #6658.

When there is no service pack, the
Msf::Exploit::Remote::SMB#smb_fingerprint_windows_sp method returns
an empty string. But in the MS08-067 exploit, instead of check an
empty string, it checks for "No Service Pack", which causes it to
never detect the right target for Windows Server 2003 SP0.

modules/exploits/windows/smb/ms08_067_netapi.rb

@@ -879,7 +879,7 @@ class Metasploit3 < Msf::Exploit::Remote
       end
 
       # Windows 2003 SP0 is mostly universal
-      if fprint['os'] == 'Windows 2003' and fprint['sp'] == 'No Service Pack'
+      if fprint['os'] == 'Windows 2003' and fprint['sp'].empty?
         mytarget = targets[3]
       end