Given a list of strings, split each one and transform the pieces into a specified output format (Python string manipulation).
Asked in the technical phone screen stage. The task tests fluent Python string handling.
Implement normalize_strings(strings). For each input string, split on any whitespace, convert every piece to lowercase, and join the pieces with _. Return the transformed strings in their original order. An empty or whitespace-only string becomes an empty string.
def normalize_strings(strings):