[Synthetics Integration] Hide error message for empty field

[justinkambic]

Is your feature request related to a problem? Please describe.
Today, we display an error message when there's an invalid or empty value in the http timeout field.

Describe the solution you'd like
I'm thinking it might be better to hide this message since it doesn't really apply. We could just highlight the field to signify it's required.

Describe alternatives you've considered
We could also display a different message explaining that it's required, but that might not be necessary.

Additional context

Notice how in the final screenshot, the field's label is highlighted red. I think in this case we don't need the message.

[elasticmachine]

Pinging @elastic/observability-design (design)