patch 8.1.2180: Error E303 is not useful when 'directory' is empty

Problem:    Error E303 is not useful when 'directory' is empty.
Solution:   Skip the error message. (Daniel Hahler, #5067)
diff --git a/src/testdir/test_recover.vim b/src/testdir/test_recover.vim
index a9934a1..a1c7bc4 100644
--- a/src/testdir/test_recover.vim
+++ b/src/testdir/test_recover.vim
@@ -12,6 +12,12 @@
     set dir=/notexist/
   endif
   call assert_fails('split Xtest', 'E303:')
+
+  " No error with empty 'directory' setting.
+  set directory=
+  split XtestOK
+  close!
+
   set dir&
 endfunc