gnulib-tool.py: Don't fail when given a bad module name.

[Collin Funk]

When using --create-testdir with a bad module name:

$ gnulib-tool.py --create-testdir --dir aaa bad-module-name
gnulib-tool: warning: module bad-module-name doesn't exist
Traceback (most recent call last):
  File "/home/collin/.local/src/gnulib/.gnulib-tool.py", line 30, in <module>
    main.main_with_exception_handling()
  File "/home/collin/.local/src/gnulib/pygnulib/main.py", line 1382, in 
main_with_exception_handling
    main()
  File "/home/collin/.local/src/gnulib/pygnulib/main.py", line 1075, in main
    testdir.execute()
  File "/home/collin/.local/src/gnulib/pygnulib/GLTestDir.py", line 209, in 
execute
    requested_licence = requested_module.getLicense()
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'getLicense'

This patch removes None from the list of GLModule objects before using
them.

I also documented the meaning of a None return in
GLModuleSystem.find() since this problem occurred in GLImport
previously.

Collin

0001-gnulib-tool.py-Don-t-fail-when-given-a-bad-module-na.patch
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