hooks: do not crash when hook files are empty

vcsserver/hook_utils/__init__.py

@@ -170,6 +170,8 b' def check_rhodecode_hook(hook_path):'
 
 
 def read_hook_content(hook_path):
+    content = ''
+    if os.path.isfile(hook_path):
     with open(hook_path, 'rb') as f:
         content = f.read()
     return content