When reading from the GitCondDB, handle properly commits with empty taggerdate

Fixes failing CondDB related tests in LHCb (run2-patches)

See merge request !1473 (merged)

(cherry picked from commit 3e81d9eb)

• f394eea8 Only invoke date parse if the string is not empty
• 2dbf04c5 Check the objecttype of the git reference before chosing the values to create the TagInfo