vim-patch:8.1.2325: crash when using balloon with empty line

Problem:    Crash when using balloon with empty line.
Solution:   Handle empty lines. (Markus Braun)
9ae862ebba

Port Test_balloon_split() from patch 8.0.1318 for merging only.
It won't run because of the function check.

src/nvim/testdir/test_popup.vim

@@ -2,6 +2,7 @@
 
 source shared.vim
 source screendump.vim
+source check.vim
 
 let g:months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
 let g:setting = ''
@@ -755,6 +756,52 @@ func Test_popup_and_previewwindow_dump()
   call delete('Xscript')
 endfunc
 
+func Test_balloon_split()
+  CheckFunction balloon_split
+
+  call assert_equal([
+        \ 'tempname: 0x555555e380a0 "/home/mool/.viminfz.tmp"',
+        \ ], balloon_split(
+        \ 'tempname: 0x555555e380a0 "/home/mool/.viminfz.tmp"'))
+  call assert_equal([
+        \ 'one two three four one two three four one two thre',
+        \ 'e four',
+        \ ], balloon_split(
+        \ 'one two three four one two three four one two three four'))
+
+  eval 'struct = {one = 1, two = 2, three = 3}'
+        \ ->balloon_split()
+        \ ->assert_equal([
+        \   'struct = {',
+        \   '  one = 1,',
+        \   '  two = 2,',
+        \   '  three = 3}',
+        \ ])
+
+  call assert_equal([
+        \ 'struct = {',
+        \ '  one = 1,',
+        \ '  nested = {',
+        \ '    n1 = "yes",',
+        \ '    n2 = "no"}',
+        \ '  two = 2}',
+        \ ], balloon_split(
+        \ 'struct = {one = 1, nested = {n1 = "yes", n2 = "no"} two = 2}'))
+  call assert_equal([
+        \ 'struct = 0x234 {',
+        \ '  long = 2343 "\\"some long string that will be wr',
+        \ 'apped in two\\"",',
+        \ '  next = 123}',
+        \ ], balloon_split(
+        \ 'struct = 0x234 {long = 2343 "\\"some long string that will be wrapped in two\\"", next = 123}'))
+  call assert_equal([
+        \ 'Some comment',
+        \ '',
+        \ 'typedef this that;',
+        \ ], balloon_split(
+        \ "Some comment\n\ntypedef this that;"))
+endfunc
+
 func Test_popup_position()
   if !CanRunVimInTerminal()
     return