Use StringPointerValue instead of StringValue

As `StringPointerValue` properly handles nil values.

For NICs retrieved via API it's possible that the `vm_name` field is null, see
https://github.com/ubicloud/ubicloud/blob/2c7862fcbc5010d4efff4aebcb96200fd123cf1e/serializers/nic.rb#L10.

In internal/provider/private_subnet_datasource.go we do the following assignment without properly
checking for nil values.

```
"vm_name": types.StringValue(*n.VmName),
```

In this PR, we switch from `StringValue`  to `StringPointerValue` for this and related fields as it properly
handles nil values. Even though these other fields should never be null it's safer to still perform this check,
e.g. in case the Ubicloud API goes through changes.

Acceptance tests have passed: https://github.com/ubicloud/terraform-provider-ubicloud/actions/runs/12128430149

internal/provider/firewall_datasource.go

@@ -102,7 +102,7 @@ func GetFirewallsState(ctx context.Context, firewalls *[]ubicloud_client.Firewal
 	firewallsValue := datasource_vm.FirewallsValue{}
 	var firewallsValues []datasource_vm.FirewallsValue
 	firewallRulesValue := datasource_vm.FirewallRulesValue{}
-	if *firewalls != nil && len(*firewalls) > 0 {
+	if firewalls != nil && len(*firewalls) > 0 {
 		for _, f := range *firewalls {
 
 			fwRules, fwRulesDiag := GetFirewallRulesState(ctx, f.FirewallRules)
@@ -112,10 +112,10 @@ func GetFirewallsState(ctx context.Context, firewalls *[]ubicloud_client.Firewal
 			}
 
 			fw := datasource_vm.NewFirewallsValueMust(firewallsValue.AttributeTypes(ctx), map[string]attr.Value{
-				"id":             types.StringValue(*f.Id),
-				"location":       types.StringValue(*f.Location),
-				"name":           types.StringValue(*f.Name),
-				"description":    types.StringValue(*f.Description),
+				"id":             types.StringPointerValue(f.Id),
+				"location":       types.StringPointerValue(f.Location),
+				"name":           types.StringPointerValue(f.Name),
+				"description":    types.StringPointerValue(f.Description),
 				"firewall_rules": fwRules,
 			})
 			firewallsValues = append(firewallsValues, fw)

internal/provider/firewall_rule_datasource.go

@@ -94,12 +94,12 @@ func GetFirewallRulesState(ctx context.Context, firewallRules *[]ubicloud_client
 	// Firewalls
 	var firewallRulesValues []datasource_vm.FirewallRulesValue
 	firewallRulesValue := datasource_vm.FirewallRulesValue{}
-	if *firewallRules != nil && len(*firewallRules) > 0 {
+	if firewallRules != nil && len(*firewallRules) > 0 {
 		for _, r := range *firewallRules {
 			fr := datasource_vm.NewFirewallRulesValueMust(firewallRulesValue.AttributeTypes(ctx), map[string]attr.Value{
-				"id":         types.StringValue(*r.Id),
-				"cidr":       types.StringValue(*r.Cidr),
-				"port_range": types.StringValue(*r.PortRange),
+				"id":         types.StringPointerValue(r.Id),
+				"cidr":       types.StringPointerValue(r.Cidr),
+				"port_range": types.StringPointerValue(r.PortRange),
 			})
 			firewallRulesValues = append(firewallRulesValues, fr)
 		}

internal/provider/postgres_datasource.go

@@ -118,11 +118,11 @@ func GetPostgresFirewallRulesState(ctx context.Context, firewallRules *[]ubiclou
 
 	firewallRulesValue := datasource_postgres.FirewallRulesValue{}
 	var firewallRulesValues []datasource_postgres.FirewallRulesValue
-	if *firewallRules != nil && len(*firewallRules) > 0 {
+	if firewallRules != nil && len(*firewallRules) > 0 {
 		for _, r := range *firewallRules {
 			fr := datasource_postgres.NewFirewallRulesValueMust(firewallRulesValue.AttributeTypes(ctx), map[string]attr.Value{
-				"id":   types.StringValue(*r.Id),
-				"cidr": types.StringValue(*r.Cidr),
+				"id":   types.StringPointerValue(r.Id),
+				"cidr": types.StringPointerValue(r.Cidr),
 			})
 			firewallRulesValues = append(firewallRulesValues, fr)
 		}

internal/provider/private_subnet_datasource.go

@@ -115,14 +115,14 @@ func GetNicsState(ctx context.Context, nics *[]ubicloud_client.Nic) (basetypes.L
 	var diags diag.Diagnostics
 	nicsValue := datasource_private_subnet.NicsValue{}
 	var nicsValues []datasource_private_subnet.NicsValue
-	if *nics != nil && len(*nics) > 0 {
+	if nics != nil && len(*nics) > 0 {
 		for _, n := range *nics {
 			nv := datasource_private_subnet.NewNicsValueMust(nicsValue.AttributeTypes(ctx), map[string]attr.Value{
-				"id":           types.StringValue(*n.Id),
-				"name":         types.StringValue(*n.Name),
-				"private_ipv4": types.StringValue(*n.PrivateIpv4),
-				"private_ipv6": types.StringValue(*n.PrivateIpv6),
-				"vm_name":      types.StringValue(*n.VmName),
+				"id":           types.StringPointerValue(n.Id),
+				"name":         types.StringPointerValue(n.Name),
+				"private_ipv4": types.StringPointerValue(n.PrivateIpv4),
+				"private_ipv6": types.StringPointerValue(n.PrivateIpv6),
+				"vm_name":      types.StringPointerValue(n.VmName),
 			})
 			nicsValues = append(nicsValues, nv)
 		}