Use simple if statement for setting baseVersion

app/assets/javascripts/diffs/store/getters_versions_dropdowns.js

@@ -16,8 +16,13 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
   const diffHead = parseBoolean(diffHeadParam) || (!diffHeadParam && defaultMergeRefForDiffs);
   const isBaseSelected = !state.startVersion && !diffHead;
   const isHeadSelected = !state.startVersion && diffHead;
+  let baseVersion = null;
 
-  const baseVersion = {
+  if (
+    !defaultMergeRefForDiffs ||
+    (defaultMergeRefForDiffs && !state.mergeRequestDiff.head_version_path)
+  ) {
+    baseVersion = {
       versionName: state.targetBranchName,
       version_index: DIFF_COMPARE_BASE_VERSION_INDEX,
       href: state.mergeRequestDiff.base_version_path,
@@ -25,6 +30,7 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
       selected:
         isBaseSelected || (defaultMergeRefForDiffs && !state.mergeRequestDiff.head_version_path),
     };
+  }
 
   const headVersion = {
     versionName: state.targetBranchName,
@@ -45,8 +51,6 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
 
   return [
     ...state.mergeRequestDiffs.slice(1).map(formatVersion),
-    (!defaultMergeRefForDiffs ||
-      (defaultMergeRefForDiffs && !state.mergeRequestDiff.head_version_path)) &&
     baseVersion,
     state.mergeRequestDiff.head_version_path && headVersion,
   ].filter(a => a);