cask: exit successfully when given no arguments

Fixes https://github.com/Homebrew/brew/issues/3601.

Library/Homebrew/cask/lib/hbc/cli.rb

@@ -229,6 +229,7 @@ module Hbc
         purpose
         usage
 
+        return if @command.nil?
         return if @command == "help" && @args.empty?
 
         raise ArgumentError, "help does not take arguments."

Library/Homebrew/test/cask/cli_spec.rb

@@ -16,10 +16,16 @@ describe Hbc::CLI, :cask do
   it "ignores the `--language` option, which is handled in `OS::Mac`" do
     cli = described_class.new("--language=en")
     expect(cli).to receive(:detect_command_and_arguments).with(no_args)
-    expect(cli).to receive(:exit).with(1)
     cli.run
   end
 
+  context "when given no arguments" do
+    it "exits successfully" do
+      expect(subject).not_to receive(:exit).with(be_nonzero)
+      subject.run
+    end
+  end
+
   context "when no option is specified" do
     it "--binaries is true by default" do
       command = Hbc::CLI::Install.new("some-cask")