Fix demo-rollup restart issue

[citizen-stig]

Description

Adds is_empty method to Storage trait so it can be checked during initialization.

Linked Issues

• Fixes Demo rollup and prover fails after restart #340

Testing

Added unit tests with restart lifecycle

Docs

Trait documentation has been updated

[codecov]

Codecov Report

Merging #360 (59b84dc) into main (f5b4bd4) will increase coverage by 0.1%.
The diff coverage is 93.5%.

❗ Current head 59b84dc differs from pull request most recent head 5292034. Consider uploading reports for the commit 5292034 to get more accurate results

Impacted Files	Coverage Δ
full-node/db/sov-schema-db/src/lib.rs	85.3% <ø> (ø)
module-system/sov-state/src/storage.rs	100.0% <ø> (ø)
module-system/sov-state/src/zk_storage.rs	75.0% <0.0%> (-2.6%)	⬇️
module-system/sov-state/src/prover_storage.rs	95.9% <100.0%> (+0.8%)	⬆️

... and 1 file with indirect coverage changes