[BUG] NPE/IndexOutOfBoundsException when request param is null or empty

- severity: High
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/client/ShenyuGrpcClient.java:75,81`
- description: When `parameterTypes` is blank, `param` can be null and execution reaches `client.call()`. Inside `call()`, line 75 `GsonUtils.getInstance().toObjectMap(requestJsons)` with null returns null; `JsonMessage.buildJsonMessageList(null)` NPEs. Even if it returns an empty list, line 81 `jsonRequestList.get(0)` throws `IndexOutOfBoundsException`.
- impact: gRPC methods with no parameter types crash with an uncaught exception instead of being proxied.
- suggested_fix: Handle null/blank `requestJsons` by building a single default empty DynamicMessage.
- confidence: High
- related_existing: none

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Start in shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/client/ShenyuGrpcClient.java at lines 75 and 81, then trace how client.call() handles blank requestJsons and parameterTypes. Reproduce a gRPC call with no parameter types; done means it no longer throws NPE or IndexOutOfBoundsException and the method is proxied with an empty request.

Written by the indexing model from the issue text.