Bug: org-map-drill-entries fails in org-tags-expand on empty string

[Robert Irelan <rirelan@gmail.com>]

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Patch to fix attached.

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From a3af9004fbc5a8a86ba8f990d393403ec671f9bf Mon Sep 17 00:00:00 2001
From: Robert Irelan <rmirelan@google.com>
Date: Wed, 23 Jan 2019 11:11:14 -0800
Subject: [PATCH] org-drill: Fix tag match in org-map-drill-entries

* contrib/lisp/org-drill.el: Change MATCH argument to ORG-MAP-ENTRIES
from "" to nil.

Without this, org-tags-expand will throw an error on the empty string.
---
 contrib/lisp/org-drill.el | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/contrib/lisp/org-drill.el b/contrib/lisp/org-drill.el
index c7f509bff..895003395 100644
--- a/contrib/lisp/org-drill.el
+++ b/contrib/lisp/org-drill.el
@@ -1483,10 +1483,10 @@ the current topic."
          (when (and (not (org-invisible-p))
                     (> (org-current-level) drill-entry-level))
            (when (or (/= (org-current-level) (1+ drill-entry-level))
-                        (funcall test))
+                     (funcall test))
              (hide-subtree))
            (push (point) drill-sections)))
-       "" 'tree))
+       nil 'tree))
     (reverse drill-sections)))
 
 
-- 
2.20.1.321.g9e740568ce-goog