patch 8.1.1447: not allowed to create an empty popup

Problem:    Not allowed to create an empty popup.
Solution:   Remove restriction that there is some text. (closes #4470)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 6e05919..4ed3b1c 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -596,3 +596,17 @@
   call popup_close(winid, 'done')
   call assert_equal('done', g:result)
 endfunc
+
+func Test_popup_empty()
+  let winid = popup_create('', {'padding': [2,2,2,2]})
+  redraw
+  let pos = popup_getpos(winid)
+  call assert_equal(4, pos.width)
+  call assert_equal(5, pos.height)
+
+  let winid = popup_create([], {'border': []})
+  redraw
+  let pos = popup_getpos(winid)
+  call assert_equal(2, pos.width)
+  call assert_equal(3, pos.height)
+endfunc