Remove whitespace from XML input

def _join_lines(txt):
    """ Remove whitespace from XML input """
    txt = txt or ''  # Handle NoneType input values
    val = ''
    lines = txt.split('\n')
    for line in lines:
        stripped = line.strip()
        if len(stripped) == 0:
            continue
        val += stripped + ' '
    return val.strip()