Remove properties when set to an empty string

Fixes #3109.

src/TDB2.cpp

@@ -169,9 +169,14 @@ void TDB2::modify (Task& task)
       update = true;
     }
     if (update) {
+      // An empty string indicates the value should be removed.
+      if (v_new == "") {
+        tctask.set_value(k, {});
+      } else {
         tctask.set_value(k, make_optional (v_new));
       }
     }
+  }
 
   // we've now added and updated properties; but must find any deleted properties
   for (auto kv : tctask_map) {

test/tw-3109.t

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+. bash_tap_tw.sh
+
+task add emptyval
+task 1 done
+task 1 mod end: status:pending
+task_end=`task 1 info | grep ^End | sed -e 's/^End //' || true`
+echo "task_end: $task_end"
+
+# `task mod end:` should have deleted the end.
+[[ "$task_end" == "" ]]