fix: handle invalid JSON fields in cryptor plugin

[SouthwestAsiaFloat]

Fixes #6878

• Return null when the request or response body is not a JSON object.

• Validate every node in a nested field path before accessing it.

• Prevent NullPointerException, ClassCastException, and IllegalStateException when a configured field is missing or has an unexpected JSON type.

• Add unit tests covering non-object bodies and invalid nested paths.

• You have read the contribution guidelines.

• You submit test cases (unit or integration tests) that back your changes.

• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[Aias00]

PR #6960 Review — fix: handle invalid JSON fields in cryptor plugin

Verdict: APPROVE

Scope

JsonUtil.parser(String json, String fieldName) in shenyu-plugin-cryptor. Adds null/type
safety when extracting a (possibly nested) field from a JSON body.

Verification

• import java.util.Objects is already present (head file L18) — compile is fine.
• Top-level guard: GsonUtils.getInstance().toObjectMap(json) returns null for non-object
  JSON (array "[...]", bare string, malformed). Now returns null instead of propagating.
• Nested branch rewrite:
  • First segment cast to JsonObject is now guarded by instanceof (was an unchecked cast
    that could throw ClassCastException).
  • Every intermediate node is checked for null and isJsonObject() before descending.
  • Final node is checked for isJsonPrimitive() before getAsString() (was
    getAsJsonPrimitive() which threw on objects/arrays).
  • All early-exit paths return null consistently.
• Non-nested else branch is unchanged: map.get(fieldName) == null ? null : .toString().
  Callers (e.g. the Set<String> overload in parser(json, fieldNames)) already filter out
  blank/null results, so returning null here is safe and matches prior missing-field behavior.
• Logic traced against every new test case; all 8 negative cases resolve to null and both
  positive cases (name and data.nested.name) return the expected value.

Tests

JsonUtilTest added (49 lines): positive + non-object body + 6 invalid-nested-path cases.
Covers the exact exception paths this PR eliminates (NPE / CCE / IllegalStateException).

Notes

• Behavioral change: a nested path whose final node is a JSON object/array now returns null
  instead of throwing. In the cryptor context null already means "no field to process", so
  this is the intended safe fallback, not a silent data-loss.
• No public API/signature change; no new dependency.

Clean, minimal, well-tested null-safety fix. Approving.