Better guarantee that a popover position can be found

[chandlerprall]

Summary

Fixes a bug described in #2874 (comment)

The context menu's first render pass does not include any content, only the wrapping DOM. With a border, that DOM's height is 2*borderWidth and no border means no height. No height means the content isn't displayable, and the popover service errors out. This PR changes the service to be more lenient in these cases.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2948/

[snide]

Confirmed this by checking popovers that were causing problems and by making sure our other popovers still work as expected. Functionally LGTM.

[cchaos]

💯 Works for me. I tested locally in Chrome and in all the themes (0 border panels) and even removed all content from the context menu and it still rendered. Thanks!