do nothing if there are no files to download

sia-ant/job_renter.go

@@ -121,7 +121,7 @@ func (j *JobRunner) storageRenter() {
 		}
 	}()
 
-	// Every 200 seconds, download a file.  Verify that the download call
+	// Every 200 seconds, attempt to download a file.  Verify that the download call
 	// succeeds correctly, the file is placed in the download list, and the file
 	// is removed from the download list, indicating successful download
 	// completion.
@@ -143,6 +143,11 @@ func (j *JobRunner) storageRenter() {
 					log.Printf("%v jobStorageRenter ERROR: %v\n", j.siaDirectory, err)
 				}
 
+				// Do nothing if there are not any files to be downloaded.
+				if len(renterFiles.Files) == 0 {
+					return
+				}
+
 				// Filter out files which are not available.
 				availableFiles := renterFiles.Files[:0]
 				for _, file := range renterFiles.Files {