Kibana backend: Panic on no result?

[mumoshu]

Hi. First of all, thank you so much for sharing a great tool 👍

I just ran ax against an empty es index behind Kibana innocently, and it panicked like:

$ ax
Querying index circleci-builds
panic: runtime error: index out of range

goroutine 19 [running]:
github.com/egnyte/ax/pkg/backend/kibana.(*Client).queryMessages(0xc420253320, 0x18c9540, 0xc420085c40, 0xc420039610, 0xf, 0x1804218, 0x0, 0x0, 0xc42028a000, 0x0, ...)
	/Users/romanlevin/go/src/github.com/egnyte/ax/pkg/backend/kibana/query.go:136 +0x1ce9
github.com/egnyte/ax/pkg/backend/kibana.(*Client).querySubIndex(0xc420253320, 0x18c9540, 0xc420085c40, 0xc420039610, 0xf, 0x1804218, 0x0, 0x0, 0xc42028a000, 0x0, ...)
	/Users/romanlevin/go/src/github.com/egnyte/ax/pkg/backend/kibana/query.go:190 +0x97
github.com/egnyte/ax/pkg/backend/kibana.(*Client).Query.func1(0xc420253320, 0x18c9540, 0xc420085c40, 0xc420286000, 0xc420288000)
	/Users/romanlevin/go/src/github.com/egnyte/ax/pkg/backend/kibana/query.go:164 +0x127
created by github.com/egnyte/ax/pkg/backend/kibana.(*Client).Query
	/Users/romanlevin/go/src/github.com/egnyte/ax/pkg/backend/kibana/query.go:161 +0x16b

Perhaps we aren't correctly handling the case here?