Fix failing test when cssPath is undefined and there is no input filename
Fix failing test when cssPath is undefined and there is no input filename

In this scenario, the `inputFile` variable gets set to undefined. The
`path` module then becomes unhappy that a non-string is passed to it as
a parameter.

The fix included in this commit defaults the `inputFile` to an empty
string instead of undefined.

glebmachine/postcss-cachebuster,glebmachine/postcss-cachebuster