我正在尝试将excel文件转换为xml。使用这个框架代码；

wb = load_workbook("deneme.xlsx")

# Getting an object of active sheet 1

ws = wb.worksheets[0]
doc, tag, text = Doc().tagtext()

xml_header = '<?xml version="1.0" encoding="UTF-8"?>'

# Appends the String to document

doc.asis(xml_header)
with tag('userdata'):
    with tag('basicinf'):
        for row in ws.iter_rows(min_row=2, max_row=None, min_col=1, max_col=90):
            row = [cell.value for cell in row]
            a=row[0]

            with tag("usernumber"):
                text(row[0])

            with tag("username"):

                text(row[1])
            with tag("serviceareacode"):
                text(row[2])
            with tag("language"):
                text(row[3])
            with tag("welcomemsgid"):
                text(row[4])
            with tag("calledlimitedid"):
                text(row[5])
            with tag("followmeflag"):
                text(row[6])
            with tag("followmenumber"):
                text(row[7])
            with tag("mobilespecial"):
                text(row[8])
result = indent(
    doc.getvalue(),
    indentation='  ',
    indent_text=False
)

print(result)
with open("routescheme_{}.xml".format(a), "w") as f:
    f.write(result)

现在，如果我不在excel中的[0]行上写入任何输入，则会出现错误。我只是想，例如，当第[0]行为空时，它应该是 <usernumber></usernumber> 我该怎么做？