MM-64632: Fix a panic in bulk import (#33360)

We were incorrect de-referencing the channels slice
without checking for nil pointer first.

https://mattermost.atlassian.net/browse/MM-64632
```release-note
NONE
```

server/channels/app/imports/import_validators.go

@@ -791,16 +791,17 @@ func isValidGuestRoles(data UserImportData) bool {
 				gtc++
 			}
 
-			if *team.Channels != nil {
-				for _, channel := range *team.Channels {
-					if channel.Roles != nil && model.IsInRole(*channel.Roles, model.ChannelGuestRoleId) {
-						ctc++
-					}
+			if team.Channels == nil {
+				continue
+			}
+			for _, channel := range *team.Channels {
+				if channel.Roles != nil && model.IsInRole(*channel.Roles, model.ChannelGuestRoleId) {
+					ctc++
 				}
+			}
 
-				if ctc == len(*team.Channels) {
-					isChannelGuest = true
-				}
+			if ctc == len(*team.Channels) {
+				isChannelGuest = true
 			}
 		}
 		if gtc == len(*data.Teams) {

server/channels/app/imports/import_validators_test.go

@@ -1704,6 +1704,19 @@ func TestIsValidGuestRoles(t *testing.T) {
 			},
 			expected: true,
 		},
+		{
+			name: "Valid case: User with team but nil channels array",
+			input: UserImportData{
+				Roles: model.NewPointer(model.SystemUserRoleId),
+				Teams: &[]UserTeamImportData{
+					{
+						Roles:    model.NewPointer(model.TeamUserRoleId),
+						Channels: nil,
+					},
+				},
+			},
+			expected: true,
+		},
 	}
 
 	for _, tc := range testCases {