Fixed install check for git pre-release versions (#600)

May fix #599. Installer going down path of checking git, and splitting a
variable that wasn't tested for 'None`. Added guard, which fixed 3
broken unit tests.

Author: dmoore247

src/databricks/labs/ucx/install.py

@@ -776,7 +776,7 @@ def _version(self):
             dv = SemVer.parse(git_detached_version)
             datestamp = datetime.datetime.now().strftime("%Y%m%d%H%M%S")
             # new commits on main branch since the last tag
-            new_commits = dv.pre_release.split("-")[0]
+            new_commits = dv.pre_release.split("-")[0] if dv.pre_release else None
             # show that it's a version different from the released one in stats
             bump_patch = dv.patch + 1
             # create something that is both https://semver.org and https://peps.python.org/pep-0440/