[file] fix glob_paths when NO paths match, add tests.

recipe_modules/file/examples/glob.py

+# Copyright 2017 The LUCI Authors. All rights reserved.
+# Use of this source code is governed under the Apache License, Version 2.0
+# that can be found in the LICENSE file.
+
+DEPS = [
+  "file",
+  "path",
+  "json",
+]
+
+def RunSteps(api):
+  sd = api.path['start_dir']
+
+  for fname in ['thing.pat', 'other.pat', 'something', 'file']:
+    api.file.write_text("write %s" % fname, sd.join(fname), 'data')
+
+  hits = api.file.glob_paths("pat", sd, '*.pat', ['thing.pat', 'other.pat'])
+  assert hits == [sd.join('other.pat'), sd.join('thing.pat')]
+
+  hits = api.file.glob_paths("noop", sd, '*.nop', [])
+  assert hits == []
+
+  hits = api.file.glob_paths("thing", sd, '*thing*', ['thing.pat', 'something'])
+  assert hits == [sd.join('something'), sd.join('thing.pat')]
+
+
+def GenTests(api):
+  yield api.test('basic')
+