fix(repository/finalize): avoid pruning crash when base branches are not resolved yet (renovatebot#41928)

Author: Churro

lib/workers/repository/finalize/prune.spec.ts

@@ -113,6 +113,33 @@ describe('workers/repository/finalize/prune', () => {
       );
     });
 
+    it('uses defaultBranch when baseBranchPatterns exist but baseBranches are not computed yet', async () => {
+      config.branchList = [];
+      config.baseBranchPatterns = ['/^release\\/.*/'];
+      config.defaultBranch = 'main';
+      git.getBranchList.mockReturnValueOnce([
+        'renovate/release/1.x-dependency',
+      ]);
+      platform.findPr.mockResolvedValueOnce(null);
+
+      await expect(
+        cleanup.pruneStaleBranches(config, config.branchList),
+      ).resolves.not.toThrow();
+
+      expect(platform.findPr).toHaveBeenCalledExactlyOnceWith({
+        branchName: 'renovate/release/1.x-dependency',
+        state: 'open',
+        targetBranch: 'main',
+      });
+      expect(scm.isBranchModified).toHaveBeenCalledExactlyOnceWith(
+        'renovate/release/1.x-dependency',
+        'main',
+      );
+      expect(scm.deleteBranch).toHaveBeenCalledExactlyOnceWith(
+        'renovate/release/1.x-dependency',
+      );
+    });
+
     it('does nothing on dryRun', async () => {
       config.branchList = ['renovate/a', 'renovate/b'];
       GlobalConfig.set({ dryRun: 'full' });

lib/workers/repository/finalize/prune.ts

@@ -120,7 +120,7 @@ async function cleanUpBranches(
  * @param config Renovate configuration
  */
 function calculateBaseBranchRegex(config: RenovateConfig): RegExp | null {
-  if (!config.baseBranchPatterns?.length) {
+  if (!config.baseBranchPatterns?.length || !config.baseBranches?.length) {
     return null;
   }
 
@@ -130,9 +130,7 @@ function calculateBaseBranchRegex(config: RenovateConfig): RegExp | null {
     .filter(uniqueStrings)
     .map(escapeRegExp);
 
-  // calculate possible base branches and escape for regex
-  // if baseBranchPatterns is configured, baseBranches will be defined too
-  const baseBranches = config.baseBranches!.map(escapeRegExp);
+  const baseBranches = config.baseBranches.map(escapeRegExp);
 
   // create regex to extract base branche from branch name
   const baseBranchRe = regEx(