patch 8.1.0651: :args \"foo works like :args without argument

Problem:    :args \"foo works like :args without argument.
Solution:   Fix check for empty argument. (closes #3728)
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index c3f26b6..74d5642 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -2738,7 +2738,7 @@
 	    alist_new();
     }
 
-    if (!ends_excmd(*eap->arg))
+    if (*eap->arg != NUL)
     {
 	/*
 	 * ":args file ..": define new argument list, handle like ":next"