Remove empty args from argument_list in SubprocessCompiler

[solkaz]

argument_list does not check if the first element is empty, which is problematic because, by default, that is the program that will be executed by subprocess.Popen, so, for example, if the first element in argument_list is an empty string, then a PermissionError will be raised. This came up when working with django-pipeline-browserify, because the first element in the command BrowserifyCompiler passes to execute_command is BROWSERIFY_VARS, which defaults to ''. This commit will filter out empty elements in argument_list to prevent issues like this.

[cyberdelia]

Could you add a test case?

[solkaz]

Test case added

[cyberdelia]

Thanks

[frewsxcv]

Is that test failure legit?

[solkaz]

Travis CI tests seem to not be consistent, but I can test it out on some other machines.

[cyberdelia]

It's just that our build were not reliable, I fixed it in master.