qdev-monitor-test: Simplify using g_assert_cmpstr()

Use g_assert_cmpstr() instead of combining g_assert() and strcmp(3).
This simplifies the code since we no longer have to play games to
distinguish NULL from "" using "(null)".

gcc extension haters will also be happy that ?: was dropped.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

tests/qdev-monitor-test.c

@@ -32,8 +32,9 @@ static void test_device_add(void)
                    "}}");
     g_assert(response);
     error = qdict_get_qdict(response, "error");
-    g_assert(!strcmp(qdict_get_try_str(error, "desc") ?: "",
-                     "Device needs media, but drive is empty"));
+    g_assert_cmpstr(qdict_get_try_str(error, "desc"),
+                    ==,
+                    "Device needs media, but drive is empty");
     QDECREF(response);
 
     /* Delete the drive */
@@ -42,7 +43,7 @@ static void test_device_add(void)
                    "   \"command-line\": \"drive_del drive0\""
                    "}}");
     g_assert(response);
-    g_assert(!strcmp(qdict_get_try_str(response, "return") ?: "(null)", ""));
+    g_assert_cmpstr(qdict_get_try_str(response, "return"), ==, "");
     QDECREF(response);
 
     /* Try to re-add the drive.  This fails with duplicate IDs if a leaked
@@ -53,8 +54,7 @@ static void test_device_add(void)
                    "   \"command-line\": \"drive_add pci-addr=auto if=none,id=drive0\""
                    "}}");
     g_assert(response);
-    g_assert(!strcmp(qdict_get_try_str(response, "return") ?: "",
-                     "OK\r\n"));
+    g_assert_cmpstr(qdict_get_try_str(response, "return"), ==, "OK\r\n");
     QDECREF(response);
 
     qtest_end();