Don't generate `<template>` tag if it's not necessary

Currently the helper generates `<turbo-stream>` elements always with the `<template>` tag, we should just generate it when it's actually needed.

For example, this call:
```ruby
turbo_stream.reset_form("#form")
```

Currently generates:
```html
<turbo-stream action="reset_form" targets="#form"><template></template></turbo-stream>
```