input: Write a function to verify if a given string matches the pattern for all numeric strings. The function should return a boolean value indicating whether the string is a valid numeric string or not. If the string is empty, it should return False.
  assert is_numeric_string("123456789")==True
  assert is_numeric_string("abc123")==False
  assert is_numeric_string("123.456")==False
  assert is_numeric_string("")==False
output: def is_numeric_string(s):
      if not s:
          return False
      for char in s:
          if not char.isdigit():
              return False
      return True