fix(graphql): handle nullable response fields that can occur when there are server errors

[andrewazores]

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See https://github.com/cryostatio/cryostat3/issues/389 Based on #1245

Description of the change:

Adds handling of potentially null/undefined GraphQL API response fields.

Motivation for the change:

GraphQL responses may contain null/undefined fields when execution errors occurred. Notification popups appear to inform the user of these errors, but the views invoking these API calls should not render an overall broken error view state. It's better to render what content was available from the response, if any, rather than an overall broken view when only some subset of the data may not be available.

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... sh smoketest.sh...
2. ...

[github-actions[bot]]

This PR/issue depends on:

• cryostatio/cryostat-web#1245 By Dependent Issues (🤖). Happy coding!