check_yaml should not require any arguments.

pre_commit_hooks/check_yaml.py

diff --git a/pre_commit_hooks/check_yaml.py b/pre_commit_hooks/check_yaml.py
index 210c6d6..c297ccf 100644
--- a/pre_commit_hooks/check_yaml.py
+++ b/pre_commit_hooks/check_yaml.py
@@ -9,7 +9,7 @@
 @entry
 def check_yaml(argv):
     parser = argparse.ArgumentParser()
-    parser.add_argument('filenames', nargs='+', help='Filenames to check.')
+    parser.add_argument('filenames', nargs='*', help='Filenames to check.')
     args = parser.parse_args(argv)
 
     retval = 0

tests/meta_test.py

diff --git a/tests/meta_test.py b/tests/meta_test.py
new file mode 100644
index 0000000..773bb0d
--- /dev/null
+++ b/tests/meta_test.py
@@ -0,0 +1,12 @@
+
+import subprocess
+
+from pre_commit.clientlib.validate_manifest import load_manifest
+
+
+def test_all_hooks_allow_no_files():
+    manifest = load_manifest('hooks.yaml')
+
+    for hook in manifest:
+        if hook['id'] != 'pyflakes':
+            subprocess.check_call([hook['entry']])