[Uptime] Synthetics http timeout field listed as optional

[justinkambic]

Kibana version:
7.13

Elasticsearch version:
7.13

Server OS version:
macOS big sur

Browser version:
chrome 90

Browser OS version:
macOS

Original install method (e.g. download page, yum, from source, etc.):
from source

Describe the bug:
The timeout field in the synthetics integration is listed as Optional, but it doesn't behave as if it were optional.

Steps to reproduce:

1. Start creating a Synthetics integration
2. Remove the default timeout value
3. See an error state for an optional field

Expected behavior:
The field should not trigger a warning

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[justinkambic]

Looking good, although I'm think it'd be nice if we had a different error message. Or we just display an error message when the value is invalid, otherwise just leave the field highlighted if it's empty. I created elastic/uptime#343 to discuss.