Handle a null ExecutionResult in `GraphqlService`

[ikhoon]

Motivation:

If GraphQL.execute(input) returns a CompletableFuture completing exceptionally, NullPointException is raised while handling ExecutionResult.

java.lang.NullPointerException: Cannot invoke "graphql.ExecutionResult.getData()" because "executionResult" is null
    at com.linecorp.armeria.server.graphql.DefaultGraphqlService.lambda$execute$1(DefaultGraphqlService.java:117)
    at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934)
    at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911)

Modifications:

• Check if cause != null before accessing executionResult in the callback of executeGraphql(ctx, input).

Result:

• NullPointerException is no longer raised when GraphqlService handles errors.
• Closes #5815

[github-actions[bot]]

🔍 Build Scan® (commit: 3d3089a5d91d46e42a48cdef83bac9aabad1b7e7)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/bzoe7cnl6iclu
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/hnqvh7jkxcnqy
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/uq2rojimb76sa
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/lwcxd55dqboqi
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/xiivgsb7tf2cs
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/cmlgb3wmonpou
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/7ebqix3x6jf5k
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/pu7ubnondl54m