Make error for empty config a little friendlier

In the setup guide in the README, we have people start out with an empty
file (created using `touch`). Before this patch, Dotbot gave the
following error:

    Configuration file must be a list of tasks

Instead, with this patch, Dotbot says:

    Configuration file is empty, no work to do

This change was prompted by
https://github.com/anishathalye/dotbot/pull/226.

dotbot/cli.py

@@ -74,6 +74,9 @@ def main():
             log.error('No configuration file specified')
             exit(1)
         tasks = read_config(options.config_file)
+        if tasks is None:
+            log.warning('Configuration file is empty, no work to do')
+            tasks = []
         if not isinstance(tasks, list):
             raise ReadingError('Configuration file must be a list of tasks')
         if options.base_directory:

test/tests/config-empty.bash

@@ -1,7 +1,7 @@
-test_description='empty config disallowed'
+test_description='empty config allowed'
 . '../test-lib.bash'
 
-test_expect_failure 'run' '
+test_expect_success 'run' '
 run_dotbot <<EOF
 EOF
 '