Fix UnsupportedOperationException because of ImmutableCollection.add

[nbali]

java.lang.UnsupportedOperationException
	at org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:268)
	at org.apache.beam.runners.dataflow.DataflowRunner.run(DataflowRunner.java:1341)
	at org.apache.beam.runners.dataflow.DataflowRunner.run(DataflowRunner.java:198)
	at org.apache.beam.sdk.Pipeline.run(Pipeline.java:321)
	at org.apache.beam.sdk.Pipeline.run(Pipeline.java:307)

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[Abacn]

Thanks for the fix. This was introduced in #28621. I am trying to understand why the unit test did not capture it, possibly ExperimentalOption wasn't Immutable Collection there. In contrast, what is your use case making ExperimentalOption Immutable at this code path?

[Abacn]

I see, it was because the line was

firstNonNull(options.getExperiments(), new ArrayList<>());

and at time it was evaluated options.getExperiments() was null

[nbali]

Check the other uses of experiments. It's immutable everywhere. I don't know why exactly, but I'm just following that convention.

[Abacn]

Thanks for the fix! verified it works