Write a function to find vowels and consonants in a given word; ensure you account for edge cases.
Implement count_vowels_consonants(word). Return a dictionary with lowercase keys vowels and consonants, containing counts. Treat a, e, i, o, and u as vowels, count other alphabetic characters as consonants, and ignore digits, whitespace, punctuation, and symbols. The function must be case-insensitive and return zero counts for an empty string.
def count_vowels_consonants(word):