Fixes an import error when using S3/Minio with no redis (#1224)

## Context

Error is caused due to these steps:
- File is uploaded to Home server and attempts to import
- Import ends up in `claimDocument` in `HostedStorageManager`
- Tries to read doc metadata from DocWorkerMap, gets 'unknown' as md5 hash
- Thinks local doc is out of date and erases it.
- Downloads a non-existent file from S3, so import fails as it has no data.

## Proposed solution

This fixes it by checking for DummyDocWorker's special 'unknown' MD5, forcing an S3 check.

## Related issues

https://community.getgrist.com/t/no-metadata-for-imported-grist-document/6029/32

app/gen-server/lib/DocWorkerMap.ts

@@ -580,8 +580,10 @@ let dummyDocWorkerMap: DummyDocWorkerMap|null = null;
 
 export function getDocWorkerMap(): IDocWorkerMap {
   if (process.env.REDIS_URL) {
+    log.info("Creating Redis-based DocWorker");
     return new DocWorkerMap();
   } else {
+    log.info("Creating local/dummy DocWorker");
     dummyDocWorkerMap = dummyDocWorkerMap || new DummyDocWorkerMap();
     return dummyDocWorkerMap;
   }