sanity check for issue with empty pages

[mromanello]

Problem:

There may be cases where (mistakenly) an issue does not contain any pages (issue['i'] == []). This inconsistency is currently not spotted by schema validation or existing sanity checks.

Examples

Many examples in newspaper oecaen. Some issue IDs: oecaen-1918-01-04-a, oecaen-1918-07-11-a, oecaen-1918-10-28-a, oecaen-1918-12-30-a.

Behaviour:

Issue with no pages are currently skipped by the text rebuild script, but this issue usually indicates a bug in the data/code of the text importer, thus it's important to flag it.