Implement a function to find the first non-repeating character in a stream of audio metadata packets.
Treat packets as an ordered list of strings and process characters in packet order. Return the first character whose total frequency in the complete stream is one, or "" if every character repeats.
Input/output: packets is a list of strings, and the function returns a string containing one character or an empty string.
def find_first_non_repeating(packets):