def normalize(s):
    """
    Normalize strings to space joined chars.

    Args:
        s: a list of strings.

    Returns:
        A list of normalized strings.
    """
    if not s:
        return s
    normalized = []
    for ss in s:
        tokens = [c for c in list(ss) if len(c.strip()) != 0]
        normalized.append(' '.join(tokens))
    return normalized