Fix crash in FastFilesCompleter with no prefix

[evanunderscore]

Fixes #2748. This fixes the crash without altering the existing behavior.

[coveralls]

Coverage remained the same at 91.805% when pulling 4ead5c7 on evanunderscore:fix-filescompleter into 181bd60 on pytest-dev:master.

[nicoddemus]

Could you add a quick test to avoid someone introducing a regression by accident in the future? I believe just creating a FastFilesCompleter, call it directly with an empty prefix and asserting the result would be enough.

[evanunderscore]

@nicoddemus Thanks for prompting me to write a test - I discovered my fix was actually incorrect. I found a test case that was already testing the completer, so have added an empty string to its inputs.

[coveralls]

Coverage remained the same at 91.805% when pulling 6cf515b on evanunderscore:fix-filescompleter into 549f5c1 on pytest-dev:master.