fix(storage): avoid panic when pruning headers without static files

[mukminn]

Summary

Avoid a potential panic in DatabaseProvider::remove_blocks_above when no header static files exist yet.

Motivation

The code currently uses �xpect(...) on get_highest_static_file_block(Headers), which can panic in a fresh/static-file-less environment. This change makes the operation a no-op for static file pruning when there is nothing to prune.

Changes

• Handle None from get_highest_static_file_block(Headers) by skipping header pruning.
• Add a regression test to ensure the method does not panic in this scenario.

[mukminn]

CI is green. This avoids a potential panic when pruning headers in environments without any header static files yet, and includes a regression test.