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

[cockroach-teamcity]

This change is 

[craig[bot]]

Build succeeded:

• linux_amd64_fips_build
• local_roachtest
• check_generated_code
• macos_arm64_build
• macos_amd64_build
• linux_arm64_build
• acceptance
• lint
• docker_image_amd64
• unit_tests
• local_roachtest_fips
• linux_amd64_build
• windows_build
• examples_orms

[blathers-crl[bot]]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #132905: branch-release-24.3.

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