Fix _uniform_outdent failing with empty strings (issue #505)

[Crozzers]

This PR fixes #505.

_uniform_outdent attempts to find the smallest common whitespace between lines by using the min function.
If the text being passed into _uniform_outdent is empty or only consists of \n chars, then there will be no detected whitespace, leading to an empty sequence being passed to min, which throws an exception.

This PR adds a check within _uniform_outdent so that if no whitespace is detected, the function returns early.

[mhils]

You're beating me to it, thanks so much! 🍰

[nicholasserra]

LGTM thanks both of you!