vim-patch:8.1.2180: Error E303 is not useful when 'directory' is empty (#11257)

Problem:    Error E303 is not useful when 'directory' is empty.
Solution:   Skip the error message. (Daniel Hahler, vim/vim#5067)
00e192becd

src/nvim/memline.c

@@ -523,9 +523,9 @@ void ml_open_file(buf_T *buf)
     }
   }
 
-  if (mfp->mf_fname == NULL) {          /* Failed! */
-    need_wait_return = TRUE;            /* call wait_return later */
-    ++no_wait_return;
+  if (*p_dir != NUL && mfp->mf_fname == NULL) {
+    need_wait_return = true;  // call wait_return later
+    no_wait_return++;
     (void)EMSG2(_(
             "E303: Unable to open swap file for \"%s\", recovery impossible"),
         buf_spname(buf) != NULL ? buf_spname(buf) : buf->b_fname);

src/nvim/testdir/test_recover.vim

@@ -14,6 +14,12 @@ func Test_recover_root_dir()
     set dir=/notexist/
   endif
   call assert_fails('split Xtest', 'E303:')
+
+  " No error with empty 'directory' setting.
+  set directory=
+  split XtestOK
+  close!
+
   set dir&
 endfunc