patch 9.1.0599: Termdebug: still get E1023 when specifying arguments

Problem:  Termdebug: still get E1023 when specifying arguments and using
          a prompt buffer.
Solution: Use empty() instead of len().  Add a test.  Fix wrong order of
          arguments to assert_equal() in Test_termdebug_basic().
          (zeertzjq)

closes: #15288

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index acb4bad..48c7f74 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    599,
+/**/
     598,
 /**/
     597,