qemu: Alter dump-guest-memory command generation

The qemuMonitorJSONMakeCommand can properly handle a NULL string
by using the "S:" parameter instead of "s:", so let's use that
of having in if/else condition that only adds the "s:".

src/qemu/qemu_monitor_json.c

@@ -3170,19 +3170,11 @@ qemuMonitorJSONDump(qemuMonitorPtr mon,
     virJSONValuePtr cmd = NULL;
     virJSONValuePtr reply = NULL;
 
-    if (dumpformat) {
-        cmd = qemuMonitorJSONMakeCommand("dump-guest-memory",
-                                         "b:paging", false,
-                                         "s:protocol", protocol,
-                                         "s:format", dumpformat,
-                                         NULL);
-    } else {
-        cmd = qemuMonitorJSONMakeCommand("dump-guest-memory",
-                                         "b:paging", false,
-                                         "s:protocol", protocol,
-                                         NULL);
-    }
-
+    cmd = qemuMonitorJSONMakeCommand("dump-guest-memory",
+                                     "b:paging", false,
+                                     "s:protocol", protocol,
+                                     "S:format", dumpformat,
+                                     NULL);
     if (!cmd)
         return -1;