[PR] Fix TopicUtil accepting shared subscription with empty filter [bifromq]

["pallavithegod (via GitHub)" <gi...@apache.org>]

pallavithegod opened a new pull request, #317:
URL: https://github.com/apache/bifromq/pull/317

   ## Summary
   Reject `$share/g/` and `$oshare/g/` where the filter part is empty.
   
   Previously `TopicUtil.isValidTopicFilter` returned `true` for these inputs because after skipping the share-name separator, `i` was advanced to `topicFilter.length()` and the main scan loop was skipped entirely.
   
   ## Fix
   Add explicit empty-filter guard after skipping the separator, per MQTT-4.8.2-2.
   
   Fixes #288
   
   ## Verification
   - Updated `TopicUtilsTest`: `$share/g/` now asserts false, added `$oshare/g/` coverage. `$share/g//` remains true.
   - `TopicUtilsTest`: 7 tests run, 0 failures.


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