fix(GlFilteredSearch): Keep active state on backspace on first token

What does this MR do?

This is a follow-up to !2705 (merged), which changed how the active state of tokens is managed on token destruction.

It introduced a small usability issue where if you repeatedly backspace to clear GlFilteredSearch of all tokens, you could no longer enter new tokens (though you could enter text), and you had to blur and then focus the component to fix it.

This change makes sure that if you backspace-to-delete the first token, then the new first token will be activated.

Screenshots

Before	After
fs_before	fs_after

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.