Remove extra space above empty messages on vulnerability report table

What does this MR do and why?

On the vulnerability report, we show 2 types of empty messages in the vulnerability list:

1. When the project has no vulnerabilities

2. When the project has vulnerabilities, but the filters returned no results

These messages use the GlEmptyState component. There is a class that adds a top margin that looks correct when the component is shown by itself on a page, but causes the spacing to be off when it's shown inside a table:

This MR fixes the spacing so that the space above and below the message are the same.

Before	After

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