client-go remotecommand: avoid "unexpected error" log when there is no error

Normal execution always started to trigger this after adding more logging in
the E2E framework's exec_util.go:

     I0506 21:23:56.781188 6341 exec_util.go:201] unexpected error trying to use websockets for pod exec: <nil>

That the "should fall back" implementation gets called when there is no error
and thus no reason to even consider falling back is odd. Now the execute
implementation checks for nil first.

This is the same approach taken also in the portforward code:
b35c5c0a30/staging/src/k8s.io/client-go/tools/portforward/fallback_dialer.go (L52)

staging/src/k8s.io/client-go/tools/remotecommand/fallback.go

@@ -52,7 +52,7 @@ func (f *FallbackExecutor) Stream(options StreamOptions) error {
 // initial primary call to upgrade to a websocket connection fails.
 func (f *FallbackExecutor) StreamWithContext(ctx context.Context, options StreamOptions) error {
 	err := f.primary.StreamWithContext(ctx, options)
-	if f.shouldFallback(err) {
+	if err != nil && f.shouldFallback(err) {
 		klog.V(4).Infof("RemoteCommand fallback: %v", err)
 		return f.secondary.StreamWithContext(ctx, options)
 	}