Add padding to mobile nav menu

[ryankeairns]

Describe the bug
Looks like we might have lost some padding in the mobile nav.

From EUI docs

From Kibana > Discover

Impact and severity
The impact and severity are likely to be low as we see low, single-digit use on smaller viewports. That said, we don't know who/how this is used outside of our walls.

Environment and versions

• EUI version:
• React version:
• Kibana version (if applicable): main branch (8.16)
• Browser: arc
• Operating System: mac

To Reproduce
Steps to reproduce the behavior:

1. Go to https://eui.elastic.co/#/layout/header/elastic-pattern and open the 'Fullscreen demo' or run Kibana locally and open Discover with a small viewport

Expected behavior
Padding within the panel... something like:

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Screenshots
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Additional context
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[cee-chen]

@ryankeairns it looks like the lack of padding has been the case / intentional since the inception of the component: https://eui.elastic.co/v40.0.0/#/layout/header/elastic-pattern

If we'd like to change it, that's fine, but at that point EuiButtonEmptys may look a tad odd and like they have too much spacing.

[cee-chen]

@ryankeairns just to confirm, do you want to go ahead and add the padding (with a note that EuiButtonEmptys may then look very spaced out?)

[ryankeairns]

Hey, apologies, I was out. I would vote for adding the padding as it will result in a nicer default than how it looks today... even if still imperfect. ty