Providers without metadata service or serverInfo shouldn't break dashboard (#1761)

- Fix a number of issues that arose while testing a provider without a metadata service or serverInfo object returned via DMP calls. These should be optional services/features and we should be resilient to them not existing. In most places we already have these checks
- This does not cover a number of "improvement" scenarios, such as filtering extension tabs by provider, and defaulting any tabs that don't specify a provider to be MSSQL. This and some other features to ensure things make sense will be implemented in separate PRs but this unblocked the scenario

src/sql/services/metadata/metadataService.ts

@@ -63,7 +63,7 @@ export class MetadataService implements IMetadataService {
 			}
 		}
 
-		return Promise.resolve(undefined);
+		return Promise.resolve([]);
 	}
 
 	public getTableInfo(connectionUri: string, metadata: sqlops.ObjectMetadata): Thenable<sqlops.ColumnMetadata[]> {