input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count. The function should return the compressed string. If the given string is empty, the function should return an empty string.
  assert compress("aabbbbbbcccc") == "2a6b4c"
  assert compress("aaabbcc") == "3a2b2c"
  assert compress("abc") == "1a1b1c"
  assert compress("") == ""
output: def compress(s):
      if not s:
          return ""
      compressed = ""
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i - 1]:
              count += 1
          else:
              compressed += str(count) + s[i - 1]
              count = 1
      compressed += str(count) + s[-1]
      return compressed