Make sure (*Struct).Fields map is not nil before assigment

[kirecek]

During the following operation:

				out.Fields[trimmedKey] = &types.Value{
					Kind: &types.Value_StringValue{
						StringValue: trimmedVal,
					},
				}

fields map is not initialized and panics, i.e:

panic: assignment to entry in nil map

goroutine 2747 [running]:
github.com/gravitational/teleport/api/types/events.(*Struct).trimToMaxSize(0xc00370f5c0, 0x2dc4)
    github.com/gravitational/teleport/api@v0.0.0/types/events/events.go:195 +0x5d0
github.com/gravitational/teleport/api/types/events.(*AppSessionDynamoDBRequest).TrimToMaxSize(0xc002b3c848, 0x10000)
    github.com/gravitational/teleport/api@v0.0.0/types/events/events.go:811 +0x4ee
github.com/gravitational/teleport/lib/events.(*ProtoStream).RecordEvent(0xc00307bd40, {0x55740285e470, 0xc0032bfc70}, {0x55740275f7c0?, 0xc003908970?})
    github.com/gravitational/teleport/lib/events/stream.go:395 +0x9e
github.com/gravitational/teleport/lib/events.(*SessionWriter).processEvents(0xc00307be60)
    github.com/gravitational/teleport/lib/events/session_writer.go:435 +0x336
github.com/gravitational/teleport/lib/events.NewSessionWriter.func1()
    github.com/gravitational/teleport/lib/events/session_writer.go:61 +0x1c
created by github.com/gravitational/teleport/lib/events.NewSessionWriter in goroutine 2744
    github.com/gravitational/teleport/lib/events/session_writer.go:60 +0x33b

This occurs for example in DynamoDB queries.

I added very simple test case that covers trimToMaxSize for given type.

changelog: fix panic when trimming audit log entries

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[kirecek]

I have read the CLA Document and I hereby sign the CLA

[capnspacehook]

recheck

[capnspacehook]

Thanks for the PR! I thought this was covered already in lib/events.TestTrimToMaxSize but I guess Struct objects were not properly populated so the test never trimmed them. Will add that in a followup PR.

[kirecek]

Yes, Struct mostly does not implement AuditEvent interface covered by the TestTrimToMaxSize, therefore I took easier/faster path and implemented test only for this particular struct 🙈

All checks in gh workflows failed tho, rebased to master again.

[capnspacehook]

@kirecek it looks like some of our workflows won't run when the PR is created by an external contributer, so I created a PR here that has your changes: #53035. I made some small modifications to clean up the Struct trimming logic. Thanks for the PR!

[kirecek]

cool, thank you! Closing this then :)