wasm: Context::sendLocalResponse() can return Ok without doing anything

The current implementation of Envoy::Extensions::Common::Wasm::Context::sendLocalResponse does nothing in the case where `decoder_callbacks_` is null [here](https://github.com/envoyproxy/envoy/blob/4189535918f7e96106d8c196b885dc8d54a57dbc/source/extensions/common/wasm/context.cc#L1622).

In practice, we've found that error case is easy to hit if the call to `sendLocalResponse()` is performed in a callback where the effective context is not a stream context. A concrete example is a gRPC callout, where response callbacks do not automatically set the effective context to the stream context present when the call was initiated. Although one can address this failure by having plugin code call `setEffectiveContext()` prior to `sendLocalResponse()`, it's not at all clear to the plugin that this is needed since without it, `sendLocalResponse()` silently fails and returns Ok.

Proposed fix is to add an `else` clause [here](https://github.com/envoyproxy/envoy/blob/4189535918f7e96106d8c196b885dc8d54a57dbc/source/extensions/common/wasm/context.cc#L1642) that returns `NotFound`, `InternalFailure`, or some new enum value of `WasmResult` in this case.