Fix segfault when bindsym command is empty (#5001)

Remove end token from BINDCOMMAND

fixes i3/i3#5000: bindsym command cannot be empty, because NULL string
causes i3 to segfault when copied in configure_binding()

testcases/t/546-empty-bindcommand.t

@@ -0,0 +1,31 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test that i3 doesn't crash if the binding command is empty.
+# Ticket: #5000
+
+use i3test i3_autostart => 0;
+
+my $config = <<EOT;
+# i3 config file (v4)
+bindsym X
+EOT
+
+my $pid = launch_with_config($config);
+does_i3_live;
+
+exit_gracefully($pid);
+done_testing;