patch 9.1.0098: CompletionChanged not triggered when new leader added without matches

Problem:  CompletionChanged not triggered when new leader added causing
          no matching item in the completion menu
Solution: When completion is active but no items matched still trigger
          CompletChanged event
          (glepnir)

closes: #13982

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 879d1fa..f5cb8b2 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -1139,6 +1139,10 @@
     let g:event = copy(v:event)
     let g:item = get(v:event, 'completed_item', {})
     let g:word = get(g:item, 'word', v:null)
+    let l:line = getline('.')
+    if g:word == v:null && l:line == "bc"
+      let g:word = l:line
+    endif
   endfunction
   augroup AAAAA_Group
     au!
@@ -1158,6 +1162,8 @@
   call assert_equal(v:null, g:word)
   call feedkeys("a\<C-N>\<C-N>\<C-N>\<C-N>\<C-P>", 'tx')
   call assert_equal('foobar', g:word)
+  call feedkeys("S\<C-N>bc", 'tx')
+  call assert_equal("bc", g:word)
 
   func Omni_test(findstart, base)
     if a:findstart