fix: Conditional slot rendering for use in modals

What does this MR do?

In gitlab!46055 (merged) we discovered that when we use GlFormInputGroup in a modal with a copy button inside the <template #append>, the copy button is visible when the modal is opened the first time, but it is no longer there when the modal is opened a second time.

Using v-if="$scopedSlots.append" instead of v-if="$slots.append" in GlFormInputGroup makes it so the contents of the the <template #append> is visible no matter how many times the modal is opened and closed.

The empty state component's #description slot and the #help slots in checkboxes and radio groups are conditionally rendered in the same way, so if we use them in modals in the future, this MR also helps us not to run into the same problem again.

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