Fixed error in 'Good Practices' code snippet

[anhiga]

When initializing with pytest_args = [], if no arguments are passed, shlex.split fails parsing the empty list.
It can be easily fixed by initializing with pytest_args = ''.

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[nicoddemus]

Thanks, appreciate it!

[coveralls]

Coverage remained the same at 91.805% when pulling 2a99d82 on anhiga:trivial_error into 9b2753b on pytest-dev:master.

[RonnyPfannschmidt]

good find, thanks for the fix 👍