patch 8.2.1578: Vim9: popup_clear() does not take "true" as argument

Problem:    Vim9: popup_clear() does not take "true" as argument.
Solution:   Use tv_get_bool(). (closes #6826)
diff --git a/src/popupwin.c b/src/popupwin.c
index 053d6d3..953e76c 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -2125,7 +2125,7 @@
     int force = FALSE;
 
     if (argvars[0].v_type != VAR_UNKNOWN)
-	force = (int)tv_get_number(&argvars[0]);
+	force = (int)tv_get_bool(&argvars[0]);
     close_all_popups(force);
 }