Improve Error Message for GIT APIs and prevent NPE if required credentials are missing

Current Situation

Currently a NPE is raised if credentials from the Git Management are missing. 

Desired Behavior

If one or more required Credentials are missing, an error should be raised, which states the missing credential(s).

Patches

• Patches for releases 2.5.8 and 2.6.5 are available for Linux and Windows.
  • https://download.sos-berlin.com/patches/2.5.8-patch/js7_joc_2.5.8-PATCH.API-3.JOC-1755.jar [sha256] [sig] [tsr]
  • https://download.sos-berlin.com/patches/2.6.5-patch/js7_joc_2.6.5-PATCH.API-3.JOC-1755.jar [sha256] [sig] [tsr]