Remove null check after GetPullRequestMergeBase

This method now throws `NotFoundException` rather than returning null.

Author: jcansdale

src/GitHub.InlineReviews/Services/PullRequestSessionService.cs

@@ -127,12 +127,8 @@ public async Task<string> GetPullRequestMergeBase(ILocalRepositoryModel reposito
             var baseRef = pullRequest.Base.Ref;
             var headRef = pullRequest.Head.Ref;
             mergeBase = await gitClient.GetPullRequestMergeBase(repo, baseUrl, headUrl, baseSha, headSha, baseRef, headRef);
-            if (mergeBase != null)
-            {
-                return mergeBaseCache[key] = mergeBase;
-            }
 
-            throw new FileNotFoundException($"Couldn't find merge base between {baseSha} and {headSha}.");
+            return mergeBaseCache[key] = mergeBase;
         }
 
         /// <inheritdoc/>