In commit f2ce81c, support for multiple path splitters was added. The
type of SplitPath changed from string to []string, and splitPos was
changed to loop through all values in SplitPath.
Before that commit, if SplitPath was empty, strings.Index returned 0 and
PATH_INFO was set correctly in buildEnv.
Currently, however, splitPos returns -1 for empty values of SplitPath,
behaving as if a split position could not be found at all. PATH_INFO is
then never set in buildEnv and remains empty.
Restore the old behaviour by explicitly checking whether SplitPath is
empty and returning 0 in splitPos.
Closes#3490
fa4cdde7d8