input: Write a function that takes a list of strings and returns the concatenated result of all the elements with a specified separator. Each element in the list should be joined using the separator provided. If the list is empty, the function should return an empty string. If no separator is provided, a space should be used as the default separator.
  
  assert concatenate_strings(["Sample","Text","Hello"], "/")=="Sample/Text/Hello"
  assert concatenate_strings(["Sample","Text","Hello"])=="Sample Text Hello"
  assert concatenate_strings([], "-")==""
  assert concatenate_strings(["SingleElement"])=="SingleElement"
output: def concatenate_strings(strings, separator=" "):
      return separator.join(strings)