Remove empty cat filter from theme legend

[dbauszus-glx]

Having a current filter, then toggling a category filter on and off will crash the wkt query because the check on the empty filter passes without checking for the key length.

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[simon-leech]

Tested using filter.current and toggling the styling using both the cat filter itself and the switch all method and no errors and all working as expected.