chore: fix this.rootSpan undefined bug in PageLoadInstrumentation #19022

Job	Run time
path-filter	10s
server-spotless	0s
server-unit-tests	0s
client-prettier / prettier-check	2m 22s
client-unit-tests / client-unit-tests	8m 3s
client-build / client-build	10m 19s
client-lint / client-lint	5m 7s
client-check-cyclic-deps / check-cyclic-dependencies	1m 18s
qc-result	1s
	27m 20s

Provide feedback