[BUGFIX] Fix initial file import in EM

The path calculation of the source path contains
double slashes, which is an invalid path for
the isAllowedAbsPath check, thus no files
are copied.

Resolves: #57312
Releases: 6.2
Change-Id: I1840cfe580efbdbeedd94c8a7e5469b94e70fa30
Reviewed-on: https://review.typo3.org/28802
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack