Exclude unused pages from buffer cache monitoring

[seanlinsley]

Followup to #633. As mentioned in the documentation:

There is one row for each buffer in the shared cache. Unused buffers are shown with all fields null except bufferid. Shared system catalogs are shown as belonging to database zero.

The null values cause a type conversion error:

Scan error on column index 0, name "reldatabase": converting NULL to uint64 is unsupported

We might want to track them in the future, but for now this PR excludes them to work around the issue.