Fix #3008: reject trailing data after top-level null

[ErnestHysa]

Fixes #3008.

Both Gson.fromJson and JsonParser.parseReader accepted trailing data after a top-level null because the trailing-data check was skipped when the parsed element was null.

Changes

• Gson.java assertFullConsumption(): Removed the obj != null guard so trailing-data validation runs even when the parsed object is null.
• JsonParser.java parseReader(): Removed the !element.isJsonNull() guard so trailing-data validation runs unconditionally.

Now null extra correctly throws JsonSyntaxException: Did not consume the entire document (or the equivalent from Gson.fromJson).

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[Marcono1234]

Can you please explain what the purpose of this PR here is, or the motivation?

#3008 is a pull request not an issue, and it already tries to fix the issue while also handling corner cases and extending test coverage. It just hasn't been merged yet.

[ErnestHysa]

@Marcono1234 Fair point — I missed #3008 before opening this. Looking at it now, the patch is essentially the same change to the two call sites, plus it handles the EOFException corner case (empty/whitespace-only input) and adds test coverage. My #3028 is a strict subset of #3008 with no added value, so I'm closing this PR in favor of #3008. Sorry for the noise.