x86/NMI: allow passing just "watchdog" again

This capability got inadvertently lost in commit 3ea2ba980a ("x86/NMI:
allow processing unknown NMIs when watchdog is enabled") due to an
oversight of mine.

Reported-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/nmi.c
+++ b/xen/arch/x86/nmi.c
@@ -49,6 +49,12 @@ bool_t watchdog_force = 0;
 
 static void __init parse_watchdog(char *s)
 {
+    if ( !*s )
+    {
+        opt_watchdog = 1;
+        return;
+    }
+
     switch ( parse_bool(s) )
     {
     case 0: