Fix duplicate request and use new template for vuln report filters

What does this MR do?

This MR does the following:

1. In !58523 (merged), a duplicate GraphQL request was fixed, but !57702 (merged) inadvertently re-introduced the bug. This MR changes the filter prop default value to null to re-fix the bug.

2. In the button for GlDropdown, it previously looked something like this:

<template #button-content>
  <gl-loading-icon v-if="loading"/>
  <gl-icon v-if="icon"/>
  {{ text }}
  <gl-icon name="caret-down"/>
</template>

This meant that we could either pass in some plain text and get the default icons, or we could provide a button-content template but have to add back in the icons we want. In gitlab-ui!2113 (merged), I added the ability to provide a button-text template that lets us customize the text while keeping the default icons:

<template #button-content>
  <gl-loading-icon v-if="loading"/>
  <gl-icon v-if="icon"/>
  <template #button-text>{{ text }}</template>
  <gl-icon name="caret-down"/>
</template>

This MR switches to the button-text template and removes the icons that we were manually adding back in.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is refactor work and a bugfix that already has a changelog, but was inadvertently re-broken.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done