Make default context behaves like a real context:

- when using "--context default" parameter
- when printing the list of contexts
- when exporting the default context to a tarball

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>
(+1 squashed commit)
Squashed commits:
[20670495] Fix CLI initialization for the `docker stack deploy --help` command and ensure that the dockerCli.CurrentContext() always returns a non empty context name (default as a fallback)
Remove now obsolete code handling empty string context name
Minor code cleanup

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>

cli/command/context/remove_test.go

@@ -62,3 +62,12 @@ func TestRemoveCurrentForce(t *testing.T) {
 	assert.NilError(t, err)
 	assert.Equal(t, "", reloadedConfig.CurrentContext)
 }
+
+func TestRemoveDefault(t *testing.T) {
+	cli, cleanup := makeFakeCli(t)
+	defer cleanup()
+	createTestContextWithKubeAndSwarm(t, cli, "other", "all")
+	cli.SetCurrentContext("current")
+	err := RunRemove(cli, RemoveOptions{}, []string{"default"})
+	assert.ErrorContains(t, err, `default: context "default" cannot be removed`)
+}