wasm: add missing nullptr guard for decoder/encoder callbacks. (#16609)
wasm: add missing nullptr guard for decoder/encoder callbacks. (#16609)

This PR adds missing nullptr guard for decoder/encoder callbacks on Wasm contexts.

Signed-off-by: Takeshi Yoneda <72d3043deb9ea484d21ffcca03f662fa81cbe321@tetrate.io>
lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,lyft/envoy