patch 8.1.1924: using empty string for current buffer is unexpected

Problem:    Using empty string for current buffer is unexpected.
Solution:   Make the argument optional for bufname() and bufnr().
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index a382a1a..daed3d4 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -398,10 +398,10 @@
   " make sure to use a new buffer number for x when it is loaded
   bw! x
   new
-  let a = bufnr('')
+  let a = bufnr()
   argedit x
-  call assert_equal(a, bufnr(''))
-  call assert_equal('x', bufname(''))
+  call assert_equal(a, bufnr())
+  call assert_equal('x', bufname())
   %argd
   bw! x
 endfunc