Fix crash when a background fetch prompts for credentials

This happens consistently for my when I close my MacBook's lid. It seems that
MacOS locks the user's keychain in this case, and since I have my keychain
provide the pass phrases for my ssh keys, fetching fails because it tries to
prompt me for a pass phrase.

This all worked correctly already, we have the FailOnCredentialRequest()
mechanism specifically for this situation, so all is great. The only problem was
that it was trying to pause the ongoing task while prompting the user for input;
but the task is nil for a background fetch (and should be).

pkg/commands/oscommands/cmd_obj_runner.go

@@ -331,9 +331,13 @@ func (self *cmdObjRunner) processOutput(
 		askFor, ok := checkForCredentialRequest(newBytes)
 		if ok {
 			responseChan := promptUserForCredential(askFor)
-			task.Pause()
+			if task != nil {
+				task.Pause()
+			}
 			toInput := <-responseChan
-			task.Continue()
+			if task != nil {
+				task.Continue()
+			}
 			// If the return data is empty we don't write anything to stdin
 			if toInput != "" {
 				_, _ = writer.Write([]byte(toInput))