Drop empty argument from dnsmasq call

since dnsmasq >= 2.56 now bails out with empty arguments. See
    http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=613944
for the Debian bug and
    http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=589885
for the upstream reasoning.

src/network/bridge_driver.c

@@ -477,7 +477,7 @@ networkBuildDnsmasqArgv(virNetworkObjPtr network,
     virCommandAddArgPair(cmd, "--pid-file", pidfile);
 
     /* *no* conf file */
-    virCommandAddArgList(cmd, "--conf-file=", "", NULL);
+    virCommandAddArg(cmd, "--conf-file=");
 
     virCommandAddArgList(cmd,
                          "--except-interface", "lo",