Denote if a printer is generic.

This fixes #38779.

This allows us to avoid case in which printers.GetStandardPrinter
returns nil for both printer and err removing any potential panics that
may arise throughout kubectl commands.

Please see #38779 and #38112 for complete context.

Add comment explaining adding handlers to printers.HumanReadablePrinter
also remove an unnecessary conversion of printers.HumanReadablePrinter
to printers.ResourcePrinter.

pkg/kubectl/cmd/cmd_test.go

@@ -80,8 +80,9 @@ func defaultClientConfigForVersion(version *schema.GroupVersion) *restclient.Con
 }
 
 type testPrinter struct {
-	Objects []runtime.Object
-	Err     error
+	Objects        []runtime.Object
+	Err            error
+	GenericPrinter bool
 }
 
 func (t *testPrinter) PrintObj(obj runtime.Object, out io.Writer) error {
@@ -99,6 +100,10 @@ func (t *testPrinter) AfterPrint(output io.Writer, res string) error {
 	return nil
 }
 
+func (t *testPrinter) IsGeneric() bool {
+	return t.GenericPrinter
+}
+
 type testDescriber struct {
 	Name, Namespace string
 	Settings        printers.DescriberSettings