Fix error when executing "mach doc"

[murphm8]

This should fix #4134

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3310

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[mttr]

If you get rid of the "default=" altogether, it will pass through an empty list on its own. That's probably the best solution.

edit: Also check out the first python "gotcha" here. I don't think this would have broken anything in this case, but it's something to watch out for! :)

[murphm8]

Removed the default value.

[SimonSapin]

r+

[SimonSapin]

Thanks!

[bors-servo]

saw approval from SimonSapin
at murphm8@b4171b4

[bors-servo]

merging murphm8/servo/mach_doc = b4171b4 into auto

[bors-servo]

murphm8/servo/mach_doc = b4171b4 merged ok, testing candidate = fafd3fd

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/68
success: http://build.servo.org/builders/linux2/builds/68
success: http://build.servo.org/builders/android/builds/135
success: http://build.servo.org/builders/mac1/builds/285
success: http://build.servo.org/builders/mac2/builds/285

[bors-servo]

fast-forwarding master to auto = fafd3fd