Fix: reject trailing data after top-level JSON null

[Senrian]

Summary

Fixes #3008

Gson.fromJson(String/Reader, ...) and JsonParser.parseReader(Reader) documentedly reject trailing data, but both made an exception when the parsed top-level value was JSON null.

For non-null values, trailing data already causes a JsonSyntaxException. For top-level null, the trailing-data check was skipped.

Changes

Gson.java

// Before:
if (obj != null && reader.peek() != JsonToken.END_DOCUMENT) {

// After:
if (obj == null || reader.peek() != JsonToken.END_DOCUMENT) {

JsonParser.java

// Before:
if (!element.isJsonNull() && jsonReader.peek() != JsonToken.END_DOCUMENT) {

// After:
if (jsonReader.peek() != JsonToken.END_DOCUMENT) {

Tests

Added testParseTrailingDataAfterNull() and testFromJsonTrailingDataAfterNull() in JsonParserTest.java to verify trailing data is rejected after parsing top-level null.

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