admission: do not create snapshot pacer for nil queue

[aadityasondhi]

Even though Pace() was checking for nil snapshot queue, it was asserting on the snapshotRequester interface, which would be non-nil even if the underlying queue was nil.

This patch moves the nil check outside of the pacer, and we don't create a pacer if the snapshot queue is nil.

Also, the Close() func was not doing any real work, so we remove that.

Fixes #132905.

Release note: None