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

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[kirecek]

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[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.