Fix deletion with incomplete signature metadata

[keeganmccallum]

Summary

• keep the authoritative /data/<object-id> delete when signature metadata is incomplete
• skip the secondary signature-index delete unless both instance_id and signature are unsigned numbers
• cover null and malformed metadata while preserving the normal two-delete and persistent-sync behavior

Problem

DelDataOps unconditionally deserialized instance_id and signature as numbers. Partially observed or stale distributed metadata can contain null values, causing json.exception.type_error.302 and aborting the entire delete operation before the authoritative data entry is removed.

Verification

• Regression test against the unguarded implementation aborts with type must be number, but is null.
• Focused test passes with this change for null and string values in either field, plus the valid signature-index deletion path.
• cpplint src/server/util/meta_tree.cc test/meta_tree_test.cc
• git diff --check

The separate concurrent EtcdMetaService::UpdateEndpoint member-join failure is intentionally out of scope.