XLS exporter crashes if the number of tests is 0

Steps to reproduce:

1) Run checkbox-cli --new-ui
2) Select the client-cert whitelist
3) Deselect all tests and re-select the category "Informational Tests"

Expected results:

The run finishes without errors

Actual results:

The XLS exporter crashes:

CRITICAL plainbox.crashes: Executable 'checkbox' invoked with Namespace(check_config=False, command=<checkbox_ng.commands.cli.CliCommand object at 0x7fd0ebb9fda0>, debug_console=False, debug_interrupt=False, dry_run=False, exclude_pattern_list=[], include_pattern_list=[], log_level=None, new_ui=True, not_interactive=False, pdb=False, providers=None, trace=[], whitelist=[]) has crashed
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/plainbox/impl/clitools.py", line 513, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/usr/lib/python3/dist-packages/plainbox/impl/clitools.py", line 509, in dispatch_command
    return ns.command.invoked(ns)
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/cli.py", line 59, in invoked
    self.settings).run()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 108, in run
    return self.do_normal_sequence()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 161, in do_normal_sequence
    self.export_and_send_results()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 324, in export_and_send_results
    exporter.dump(data_subset, stream)
  File "/usr/lib/python3/dist-packages/plainbox/impl/exporter/xlsx.py", line 632, in dump
    self.write_summary(data)
  File "/usr/lib/python3/dist-packages/plainbox/impl/exporter/xlsx.py", line 350, in write_summary
    self.total_pass / self.total * 100,
ZeroDivisionError: division by zero
Traceback (most recent call last):
  File "/usr/bin/checkbox-cli", line 9, in <module>
    load_entry_point('checkbox-ng==0.4.dev', 'console_scripts', 'checkbox-cli')()
  File "/usr/lib/python3/dist-packages/checkbox_ng/main.py", line 175, in checkbox_cli
    CheckBoxNGTool().main(['checkbox-cli'] + args))
  File "/usr/lib/python3/dist-packages/plainbox/impl/clitools.py", line 291, in main
    return self.dispatch_and_catch_exceptions(ns)
  File "/usr/lib/python3/dist-packages/plainbox/impl/clitools.py", line 513, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/usr/lib/python3/dist-packages/plainbox/impl/clitools.py", line 509, in dispatch_command
    return ns.command.invoked(ns)
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/cli.py", line 59, in invoked
    self.settings).run()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 108, in run
    return self.do_normal_sequence()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 161, in do_normal_sequence
    self.export_and_send_results()
  File "/usr/lib/python3/dist-packages/checkbox_ng/commands/newcli.py", line 324, in export_and_send_results
    exporter.dump(data_subset, stream)
  File "/usr/lib/python3/dist-packages/plainbox/impl/exporter/xlsx.py", line 632, in dump
    self.write_summary(data)
  File "/usr/lib/python3/dist-packages/plainbox/impl/exporter/xlsx.py", line 350, in write_summary
    self.total_pass / self.total * 100,
ZeroDivisionError: division by zero
Exception ignored in: <bound method Workbook.__del__ of <xlsxwriter.workbook.Workbook object at 0x7fd0e5232b38>>
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 127, in __del__
    self.close()
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 237, in close
    self._store_workbook()
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 420, in _store_workbook
    xlsx_file.write(os_filename, xml_filename)
  File "/usr/lib/python3.4/zipfile.py", line 1343, in write
    zinfo.header_offset = self.fp.tell() # Start of header bytes
ValueError: I/O operation on closed file
Exception ignored in: <bound method ZipFile.__del__ of <zipfile.ZipFile object at 0x7fd0ecac7d68>>