refactor: fix `Forge::processIndexes()` for empty `$this->fields` #12644

Job	Run time
Setup PHP Version for Code Coverage	1s
Others (8.3) / Sanity Tests	2m 18s
Others (8.1) / Sanity Tests	2m 34s
Others (8.2) / Sanity Tests	3m 40s
CacheLive (8.2) / Cache Live Tests	2m 11s
CacheLive (8.1) / Cache Live Tests	2m 7s
CacheLive (8.3) / Cache Live Tests	1m 56s
DatabaseLive (8.1, MySQLi, 5.7) / tests	2m 41s
DatabaseLive (8.1, MySQLi, 8.0) / tests	4m 15s
DatabaseLive (8.1, OCI8, 8.0) / tests	6m 3s
DatabaseLive (8.1, Postgre, 8.0) / tests	2m 28s
DatabaseLive (8.1, SQLite3, 8.0) / tests	2m 23s
DatabaseLive (8.1, SQLSRV, 8.0) / tests	4m 31s
DatabaseLive (8.2, MySQLi, 8.0) / tests	4m 23s
DatabaseLive (8.2, OCI8, 8.0) / tests	7m 2s
DatabaseLive (8.2, Postgre, 8.0) / tests	3m 27s
DatabaseLive (8.2, SQLite3, 8.0) / tests	3m 19s
DatabaseLive (8.2, SQLSRV, 8.0) / tests	5m 0s
DatabaseLive (8.3, MySQLi, 8.0) / tests	3m 57s
DatabaseLive (8.3, OCI8, 8.0) / tests	5m 56s
DatabaseLive (8.3, Postgre, 8.0) / tests	2m 31s
DatabaseLive (8.3, SQLite3, 8.0) / tests	3m 2s
DatabaseLive (8.3, SQLSRV, 8.0) / tests	3m 58s
SeparateProcess (8.1) / tests	3m 10s
SeparateProcess (8.2) / tests	3m 28s
SeparateProcess (8.3) / tests	3m 18s
Upload coverage results to Coveralls / coveralls	44s
	1h 30m 23s

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