Fix infinite scroll legend

What does this MR do?

This makes the GlInfiniteScroll component a bit smarter about how it renders its legend.

If the totalItems prop is not supplied (it's optional), then the component will render Showing N of 0 items, where N = fetchedItems prop value. That's one reason why this bug occurred. For more information about this, see gitlab#212528 (closed).

This simply makes the legend say Showing N items if totalItems is not a positive number.

Addresses #786 (closed).

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:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

/label devopssecure ~bug frontend /milestone %13.0