Print empty array if hosts not found

[kazeburo]

related: #324

initialize hostsFormat with make to avoid print "null" when no hosts found.

[Krout0n]

Change these expected in this test case.

mkr/hosts/app_test.go

Lines 162 to 185 in ad3a0e6

	{
	id: "name",
	hosts: []*mackerel.Host{},
	name: "Sample.app",
	expected: "null\n",
	},
	{
	id: "service",
	hosts: []*mackerel.Host{},
	service: "SampleService",
	expected: "null\n",
	},
	{
	id: "roles",
	hosts: []*mackerel.Host{},
	roles: []string{"role1", "role2"},
	expected: "null\n",
	},
	{
	id: "statuses",
	hosts: []*mackerel.Host{},
	statuses: []string{mackerel.HostStatusPoweroff, mackerel.HostStatusMaintenance},
	expected: "null\n",
	},

[Krout0n]

LGTM! Thank you for your work! @kazeburo