Create a function that takes a string s and returns a list of two-paired characters. If the string has an odd number of characters, add an asterisk * in the final pair.
See the below examples for a better understanding:
string_pairs("mubashir") ➞ ["mu", "ba", "sh", "ir"]
string_pairs("sample") ➞ ["sa", "mp", "le"]
string_pairs("airforces") ➞ ["ai", "rf", "or", "ce", "s*"]
Return [] if the given string is empty.