backend-common: workaround for database client being collected before interval

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/twenty-dolls-smoke.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-common': patch
+---
+
+Workaround for a rare race condition in tests.

packages/backend-common/src/database/DatabaseManager.ts

@@ -352,7 +352,9 @@ export class DatabaseManager {
     let lastKeepaliveFailed = false;
 
     setInterval(() => {
-      client.raw('select 1').then(
+      // During testing it can happen that the environment is torn down and
+      // this client is `undefined`, but this interval is still run.
+      client?.raw('select 1').then(
         () => {
           lastKeepaliveFailed = false;
         },