Fix warning when in detached HEAD state

Use of uninitialized value $existingBranch in scalar chomp at [...]/share/kdesrc-build/modules/ksb/Updater/Git.pm line 365.
2 jobs for work/dfaure/fix_warning in 2 minutes and 25 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #104353
smoke-test

00:01:50

passed #104352
unit-test

00:00:35