fix: make sure Fields map is not nil

Signed-off-by: Erik Jankovic <erik.jankovic@gmail.com>

Author: kirecek

api/types/events/events.go

@@ -192,6 +192,11 @@ func (m *Struct) trimToMaxSize(maxSize int) *Struct {
 		if v != nil {
 			if strVal := v.GetStringValue(); strVal != "" {
 				trimmedVal := trimStr(strVal, maxSize)
+
+				if out.Fields == nil {
+					out.Fields = make(map[string]*types.Value)
+				}
+
 				out.Fields[trimmedKey] = &types.Value{
 					Kind: &types.Value_StringValue{
 						StringValue: trimmedVal,

api/types/events/struct_test.go

@@ -0,0 +1,69 @@
+/*
+Copyright 2025 Gravitational, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package events
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/gogo/protobuf/types"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStructTrimToMaxSize(t *testing.T) {
+	testCases := []struct {
+		name    string
+		maxSize int
+		in      *Struct
+		want    *Struct
+	}{
+		{
+			name:    "Field key exceeds max limit size",
+			maxSize: 10,
+			in: &Struct{
+				Struct: types.Struct{
+					Fields: map[string]*types.Value{
+						strings.Repeat("A", 100): {
+							Kind: &types.Value_StringValue{
+								StringValue: "A",
+							},
+						},
+					},
+				},
+			},
+			want: &Struct{
+				Struct: types.Struct{
+					Fields: map[string]*types.Value{
+						strings.Repeat("A", 8): {
+							Kind: &types.Value_StringValue{
+								StringValue: "A",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := tc.in.trimToMaxSize(tc.maxSize)
+			require.True(t, reflect.DeepEqual(got, tc.want))
+		})
+	}
+}