[Core]Fix NullPointerException cause by raylet id is empty when get actor info in java worker

[larrylian]

Why are these changes needed?

Fix NullPointerException cause by raylet id is empty when get actor info in java worker.
Calling get actor info API before the Actor is successfully scheduled will result in a NullPointerException.

Related issue number

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  • This PR is not tested :(

[SongGuyang]

LGTM