feat(GlEmptyState): add svgHeight preventing content reflow

What does this MR do?

It prevents content reflow (image "pop") for the SVG in the GlEmptyState component.

I initially considered svgAspectRatio vs. svgHeight but opted for the this boring solution. In time, if we actually opt for the enum or bucketed constraints discussed here then we'd want to iterate this svgHeight solution to svgAspectRatio and likely use the padding-bottom hack.

Before:

After:

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
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  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #1001 (closed)