Fix bug when missing workbook relationship

Add a check to see if the list is empty before trying to access it's
contents. If an excel file has an overridden relationship with no word
"book" in the name it will attempt to grab the first item of an empty
list when looking up workbook relationships.

    IndexError: list index out of range

There could be a better fix to this issue I'm not well enough versed in
the xslx specification. The following xlsx file caused the issue.

    $ unzip -l some_file.xlsx
    Archive:  some_file.xlsx
      Length      Date    Time    Name
    ---------  ---------- -----   ----
          142  02-06-2024 13:28   xl/worksheets/_rels/sheet1.xml.rels
     65968555  02-06-2024 13:28   xl/worksheets/sheet1.xml
      2078037  02-06-2024 13:28   xl/sharedStrings.xml
         9867  02-06-2024 13:28   xl/styles.xml
          566  02-06-2024 13:28   xl/_rels/workbook.xml.rels
          388  02-06-2024 13:28   xl/workbook.xml
          297  02-06-2024 13:28   _rels/.rels
         1122  02-06-2024 13:28   [Content_Types].xml
    ---------                     -------
     68058974                     8 files

In `[Content_types].xml` it is overriding the relationships to point at
`_rels/.rels` rather than `xl/_rels/workbook.xml.rels`. This causes the
`workbook_relationships` list to be empty causes the error mentioned
above. One can see that it does indeed have a workbook relationship,
however it is being overridden.

`[Contenet_types].xml`:

    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
    <Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
      <Default Extension="png" ContentType="image/png"/>
      <Default Extension="jpeg" ContentType="image/jpeg"/>
      <Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>
      <Default Extension="xml" ContentType="application/xml"/>
      <Default Extension="vml" ContentType="application/vnd.openxmlformats-officedocument.vmlDrawing"/>
      <Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/>
      <Override PartName="/xl/sharedStrings.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml"/>
      <Override PartName="/xl/styles.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml"/>
      <Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/>
      <Override PartName="/_rels/.rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>
    </Types>

`xl/_rels/workbook.xml.rels`:

    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
    <Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
      <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet" Target="worksheets/sheet1.xml"/>
      <Relationship Id="rId2" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings" Target="sharedStrings.xml"/>
      <Relationship Id="rId3" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles" Target="styles.xml"/>
    </Relationships>

xlsx2csv.py

@@ -222,8 +222,11 @@ def __init__(self, xlsxfile, **options):
         self.shared_strings = self._parse(SharedStrings, self.content_types.types["shared_strings"])
         self.styles = self._parse(Styles, self.content_types.types["styles"])
         self.workbook = self._parse(Workbook, self.content_types.types["workbook"])
-        workbook_relationships = list(filter(lambda r: "book" in r, self.content_types.types["relationships"]))[0]
-        self.workbook.relationships = self._parse(Relationships, workbook_relationships)
+        workbook_relationships = list(filter(lambda r: "book" in r, self.content_types.types["relationships"]))
+        if len(workbook_relationships) > 0:
+            self.workbook.relationships = self._parse(Relationships, workbook_relationships[0])
+        else:
+            self.workbook.relationships = Relationships()
         if self.options['no_line_breaks']:
             self.shared_strings.replace_line_breaks()
         elif self.options['escape_strings']: