patch 8.1.1303: not possible to hide a balloon

Problem:    Not possible to hide a balloon.
Solution:   Hide the balloon when balloon_show() is called with an empty
            string or list.  Add balloon_gettext().
diff --git a/src/popupmnu.c b/src/popupmnu.c
index 2639d97..0b002f5 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -1154,7 +1154,10 @@
     ui_remove_balloon();
 
     if (mesg == NULL && list == NULL)
+    {
+	pum_undisplay();
 	return;
+    }
     if (list != NULL)
     {
 	listitem_T  *li;