output help only when there are no arguments

knex cli should output help only when there are no arguments.
current implementation always outputs help, even with correct arguments, such as `knex migrate:status`
the argument length should be checked via process.argv.
reference: https://github.com/tj/commander.js/#outputhelpcb

Add tests for help command

bin/cli.js

@@ -341,7 +341,7 @@ function invoke(env) {
         .catch(exit);
     });
 
-  if (!commander._args.length) {
+  if (!process.argv.slice(2).length) {
     commander.outputHelp();
   }
 

test/cli/help.spec.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const path = require('path');
+const { execCommand } = require('cli-testlab');
+
+const cliPkg = require('../../package');
+const KNEX = path.normalize(__dirname + '/../../bin/cli.js');
+
+describe('help', () => {
+  it('Prints help', () => {
+    return execCommand(`node ${KNEX} --help`, {
+      expectedOutput: 'Usage',
+    });
+  });
+
+  it('Prints help using -h flag', () => {
+    return execCommand(`node ${KNEX} -h`, {
+      expectedOutput: 'Usage',
+    });
+  });
+
+  it('Prints help when no arguments are given', () => {
+    return execCommand(`node ${KNEX}`, {
+      expectedOutput: 'Usage',
+    });
+  });
+
+  it('Does not print help when argument is given', () => {
+    return execCommand(`node ${KNEX} -V`).then(({ stdout, _ }) => {
+      expect(stdout).to.not.include('Usage');
+    });
+  });
+});

test/index.js

@@ -93,4 +93,5 @@ describe('CLI tests', function() {
   require('./cli/migrate-make.spec');
   require('./cli/seed-make.spec');
   require('./cli/version.spec');
+  require('./cli/help.spec');
 });