Handle quant_purger with empty blocklist item

Problem/Motivation

If there is an empty block item, then the getAcceptedCacheTags doesn't work because it uses the empty item as one of the block items which ends up filtering out everything in the list.

Steps to reproduce

1. Install and config quant, quant_api, quant_purger, purge, purge_ui
2. Go to /admin/config/development/performance/purge
3. Click Configure under Purge Quant
4. Click Add tag blocklist
5. Leave item empty and save
6. Run seed
7. Look in the purge_queuer_quant db table
8. Result: table is empty
9. Expected result: table has data based on the seed

Proposed resolution

Ignore empty items.

Remaining tasks

1. Update code
2. Review and test
3. Commit :)

User interface changes

API changes

Data model changes