# Obtener String vacio '' al buscar en xpath /text() cuando el nodo está vacío
# https://es.stackoverflow.com/q/76492/127
from lxml import html
cadena = '''
<body>
<td><span>$3,480.00</span></td>
<td><span>Ingreso</span></td>
<td><span>Vigente</span></td>
<td><span></span></td>
<td><span>Otro</span></td>
</body>
'''
doc = html.fromstring(cadena)
datos = [x.xpath('string()') for x in doc.xpath('//td/span')]
print(datos)
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