input: Write a python function to read keys from a text file and return them as a list of tuples.
  ```python
  assert read_keys('keys.txt') == [('key title1', 1540820240.0, 'key content line 11\nkey content line 12\n'), ('key title2', <current_time>, 'key content line 21\nkey content line 22\n')]
  assert read_keys('empty.txt') == []
  assert read_keys('invalid_format.txt') == []
output: def read_keys(filename):
      import os
      import time
  
      if not os.path.exists(filename):
          return []
  
      keys = []
      with open(filename, 'r') as file:
          content = file.read().strip()
      
      if not content:
          return keys
  
      sections = content.split('\n\n')
      for section in sections:
          lines = section.split('\n')
          if len(lines) < 3:
              return []
  
          title = lines[0].strip()
          try:
              timestamp = float(lines[1].strip())
          except ValueError:
              return []
          content = '\n'.join(lines[2:]).strip() + '\n'
          
          keys.append((title, timestamp, content))
      
      return keys