Fixed missing return types for DebugTemplate #982

Job	Run time
Run code style check (8.0)	1m 48s
Solr integration tests (7.4)	9m 43s
Solr integration tests (8.0)	9m 39s
Unit tests & SQLite integration tests (7.3)	8m 8s
Solr integration tests (8.1)	9m 30s
Unit tests & SQLite integration tests (7.4)	7m 23s
Unit tests & SQLite integration tests (8.0)	7m 25s
Unit tests & SQLite integration tests (8.1)	7m 10s
PostgreSQL integration tests (7.3)	24m 47s
MySQL integration tests (7.4)	13m 35s
PostgreSQL integration tests (7.4)	24m 54s
MySQL integration tests (8.0)	14m 35s
PostgreSQL integration tests (8.0)	21m 48s
MySQL integration tests (8.1)	13m 32s
PostgreSQL integration tests (8.1)	28m 20s
	3h 22m 17s

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