Part of the fix for #2468 - Wrong log output date

[Xoriander]

This pull request is part of the fix for getgrav/grav-plugin-admin#2468 The main part is this pull request getgrav/grav#4007

To show the date like the previous Grav versions the format 'D, d M Y H:i:s' was added. To avoid to output the current date if none date was parsed from the logfile, an if statement was added.