[BEAM-10580] Eliminate nullability errors from :sdks:java:extensions:sql:expansion-service

[jayendra13]

…sql:expansion-service
Eliminate nullability errors from :sdks:java:extensions:sql:expansion-service

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang		---		---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[kennknowles]

Looks good! Somehow the other day I thought it was already enabled. I bet this means that fixing rawtypes will catch some new errors, which is good.

[kennknowles]

It seems there is a merge commit that we don't really need here, so I will squash this PR into master.