Fix crash getting spell-check suggestions

[moffatman]

On some Samsung devices Flutter with spell-check enabled will crash when typing/moving near the ">" character.

Stack trace is

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.view.textservice.SentenceSuggestionsInfo.getSuggestionsCount()' on a null object reference
       at io.flutter.plugin.editing.SpellCheckPlugin.onGetSentenceSuggestions(SpellCheckPlugin.java:26)
       at android.view.textservice.SpellCheckerSession.lambda$handleOnGetSentenceSuggestionsMultiple$1$android-view-textservice-SpellCheckerSession(SpellCheckerSession.java:224)
       at android.view.textservice.SpellCheckerSession$$ExternalSyntheticLambda0.run(:4)
       at android.os.Handler.handleCallback(Handler.java:942)
       at android.os.Handler.dispatchMessage(Handler.java:99)
       at android.os.Looper.loopOnce(Looper.java:226)
       at android.os.Looper.loop(Looper.java:313)
       at android.app.ActivityThread.main(ActivityThread.java:8747)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:571)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1067)

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[justinmc]

Thanks for fixing this! I didn't realize this could be null.

There was a smoke test infra failure, I've rerun it.

[camsim99]

Thanks for the fix!!