[Bug]: Fix root elements not being assignable to pimcore_relation #1557

Job	Run time
setup-matrix	6s
static-analysis (8.1, lowest, false) / Static analysis with phpstan	1m 0s
static-analysis (8.3, highest, false) / Static analysis with phpstan	1m 14s
static-analysis (8.3, highest, 11.x-dev as 11.99.9, true) / Static analysis with phpstan	59s
	3m 19s

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