Fix lua_eval() routine on AttributeError exception

Found that running kill_all_servers() routine lua_eval() subroutine may
fail on exception:

  TarantoolInpector.handle() received the following error:
  Traceback (most recent call last):
    File "test-run/lib/inspector.py", line 94, in handle
      result = self.parser.parse_preprocessor(line)
    File "test-run/lib/preprocessor.py", line 73, in parse_preprocessor
      return self.lua_eval(name, expr[1:-1])
    File "test-run/lib/preprocessor.py", line 392, in lua_eval
      result = yaml.safe_load(result)
    File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 162, in safe_load
      return load(stream, SafeLoader)
    File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 112, in load
      loader = Loader(stream)
    File "/usr/local/lib/python2.7/site-packages/yaml/loader.py", line 34, in __init__
      Reader.__init__(self, stream)
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 87, in __init__
      self.determine_encoding()
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 126, in determine_encoding
      self.update_raw()
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 183, in update_raw
      data = self.stream.read(size)
  AttributeError: 'NoneType' object has no attribute 'read'

  Kill all servers ...

To let kill_all_servers() successfully complete the found exception
should be passed.

Author: avtikhon

lib/preprocessor.py

@@ -392,7 +392,10 @@ def lua_eval(self, name, expr, silent=True):
         result = self.servers[name].admin(
             '%s%s' % (expr, self.delimiter), silent=silent
         )
-        result = yaml.safe_load(result)
+        try:
+            result = yaml.safe_load(result)
+        except AttributeError:
+            result = []
         if not result:
             result = []
         return result