bdx: Reduce flicker when the list is refreshed as user is typing

* bdx.el (bdx--ivy-collection-function): Only set
`bdx--prev-candidates' when we have some existing candidates.

Author: mkcms

bdx.el

@@ -254,7 +254,8 @@ as a property of the string."
 (defun bdx--ivy-collection-function (string &rest _args)
   "Collect candidates for query STRING.
 This should be used as COLLECTION for `ivy-read'."
-  (setq bdx--prev-candidates bdx--all-candidates)
+  (when bdx--all-candidates
+    (setq bdx--prev-candidates bdx--all-candidates))
   (setq bdx--all-candidates nil)
   (setq bdx--outdated-files nil)
   (setq bdx--sources-needing-recompilation nil)