\usepackage[a4paper]{} line raises AttributeError

[sixcy]

Hello,

On version v1.1-80-g1765

Compiling a LaTeX file containing:

\usepackage[a4paper]{}

Triggers the following error:

Traceback (most recent call last):
  File "/home/csix/.local/bin/rubber", line 11, in <module>
    sys.exit(script_entry_point())
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/cmdline.py", line 336, in script_entry_point
    return Main()(sys.argv[1:])
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/cmdline.py", line 326, in __call__
    return self.main(cmdline)
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/cmdline.py", line 254, in main
    env.main.parse()
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/converters/latex.py", line 714, in parse
    self.process(self.source())
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/converters/latex.py", line 767, in process
    self.parse_file(file)
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/converters/latex.py", line 745, in parse_file
    function(self.vars, *args)
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/converters/latex.py", line 1062, in h_usepackage
    self.modules.register(name, dict)
  File "/home/csix/.local/lib/python2.7/site-packages/rubber/converters/latex.py", line 76, in register
    file.close()
AttributeError: 'NoneType' object has no attribute 'close'

Perhaps it could indicate a more helpful error message instead.

This is a very minor issue though