input: Write a function to generate a random string of a given length. The function should return the generated string. If the input length is not provided, the function should default to generating a string of 10 characters. If the input length is provided and is not an integer, the function should return an empty string.
  
  def generate_random_string(length=10):
      if not isinstance(length, int):
          return ""
      random_characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
      random_string = ''.join(random.choices(random_characters, k=length))
      return random_string
  
  assert generate_random_string(10) != generate_random_string(10) # Random strings should not be identical
  assert len(generate_random_string(15)) == 15 # Length: 15
  assert generate_random_string() == 10 # Length: Default (10)
  assert generate_random_string("a") == "" # Invalid length input
output: def generate_random_string(length=10):
      if not isinstance(length, int):
          return ""
      random_characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
      random_string = ''.join(random.choices(random_characters, k=length))
      return random_string