Change GlInfiniteScroll to not display totalItems in its legend if it wasn't supplied

This will fix the underlying cause for how this bug occurred.

Rationale:

• GlInfiniteScroll's totalItems prop is optional; unfortunately, the component always renders its "legend". Since totalItems defaults to 0, the legend in this case will say Showing n items of 0, which is precisely what caused the bug.
  • GlInfiniteScroll should change the legend to not display totalItems if it wasn't supplied.