Check early for commit on current branch

Author: gitfool

src/GitVersionCore/VersionCalculation/BaseVersionCalculators/FallbackVersionStrategy.cs

@@ -22,6 +22,10 @@ public override IEnumerable<BaseVersion> GetVersions()
         {
             Commit baseVersionSource;
             var currentBranchTip = Context.CurrentBranch.Tip;
+            if (currentBranchTip == null)
+            {
+                throw new GitVersionException("No commits found on the current branch.");
+            }
 
             try
             {